BENJ\M 


From  Excerpt  ...i/nutes  of  the  Proceedicgs  of  the 
Inolitatior  of  Civil  Engineers. 


JX  "VAN  FOSTR/lSr:>,  PUBLISHER, 
/?  MURF.AY  ANI>  ^  '  WARREN  STREET. 

1882. 


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^nj_v_u_v4            ^G  CERT/ 

Prof.  Geo. 

;  AJNU   WATH:tt  SUPPLY.     By  Prof.  W. 

Corfield,  M.  A. 
No.  18.-SEWERAGE  AND  SEWAGE   UTILIZATION, 

Prof.  W.  H.  Corfield. 
No.  19.-STRENGTH  OF  BEAMS    UNDER   TRANSVER 

LOADS.     By  Prof.  W.  Allan. 
No.  20.— BRIDGE   AND  TUNNEL  CENTRES.     By  John 

McMasters,  C.  E. 
No.  21.— SAFETY  VALVES.    By  Richard  H.  Buel,  C.  E. 


THE  VAN  NOSTBAND  SCIENCE  SERIES. 


[22. -HIGH  MASONRY  DAMS.    By  John  B  McMaster. 
,  23.-THE  FATIGUE  OF  METALS  UNDER  REPEATED 
STRAINS,  with  various  Tables  of  Results  of  Ex- 
periments.   From  the  German  of  Prof.   Ludwig 
Spangenberg.    With  a  Preface  by  S.  H.  Shreve 
i  24.— A  PRACTICAL   TREATISE   ON   THE  TEETH  OF 
WHEELS,  with  the  Theory  of  the  Use  of  Robin- 
son's Odontograph.    By  Prof.  S.  W.  Robinson. 
25.— THEORY   AND    CALCULATIONS    OF    CONTINU- 
OUS BRIDGES.     By  Mansfield  Merriman,  C.  E. 
).- PRACTICAL    TREATISE    ON   THE  PROPERTIES 
OF  CONTINUOUS  BRIDGES.    By  Charles  Bender. 
;.27.— ON  BOILER  INCRUSTATION  AND  CORROSION- 

By  F.  J.  Rowan. 
28. -ON]   TRANSMISSION     OF     POWER     BY     WIRE 

KOPES.    By  Albert  W.  Stahl. 

:9. -INJECTORS  ;  THEIR  THEORY  A$D  USE.    Trans- 
lated from  the  French  of  M.  Leon  Pouchet. 
0  —TERRESTRIAL  MAGNETISM  AND  THE  MAGNET- 
/    ISM    OF    IRON    SHIPS.       By   Prof.     Fairman 
Rogers. 

31.-THE    SANITARY   CONDITION    OF    DWELLING 
HOUSES  IN  TOWN  AND  COUNTRY.    By  George 


MAKING    FOR    SUSPENSION  BRIDGES, 
as  exemplified  in  the  construction  of  the  East 
River  Bridge.    By  Wilhelm  Hildenbrand,  C.  E. 
3.— MECHANICS  OF  VENTILATION.    By  George  W. 
Rafter,  C.  E. 

14.— FOUNDATIONS.    By  Prof.   Jules  Gaudard,   C.   E. 

Translated  from  the  French. 
:  35.— THE    ANEROID   BAROMETER:   Its  Construction 
and  Use.      Compiled  by  Prof.  G.  W.  Plympton. 
3d  Edition 

16. -MATTER  AND  MOTION.    By  J.  Clerk  Maxwell 
37.— GEOGRAPHICAL  SURVEYING  :  its  Uses,  Methods 

and  Results.    By  Frank  De  Yeaux  Carpenter, 
i  38.— MAXIMUM    STRESSES    IN  FRAMED    BRIDGES 

By  Prof.  Wm.  Cain. 

9.-A    HANDBOOK    OF   THE    ELECTROMAGNETIC 
TELEGRAPH.     By  A.  E.  Loring,  a  Practical  Tel- 
egrapher.    2d  Edition. 
[  40.— TRANSMISSION    OF    POWER   BY  COMPRESSED 

AIR.    By  Robert  Zabner,  M  E. 
41. -STRENGTH  OF  MATERIALS.    By  William  Kent. 
1 42.— VOHSSOIR  ARCHES,  applied  to  Stone  Bridges,  Tun 

nels,  Culverts  and  Domes.    By  Prof.  Wra.  Cain. 
;  43.-WAVE  AND  VORTEX  MOTION.    By  Dr.  Thomas 
Craig,  of  Johns  Hopkins  University. 


OF 


EARTHWORK. 

BY 

BENJAMIN  BAKER,  M.  Inst.  C.E 

From  Excerpt  Minutes  of  the  Proceedings  of  the 
Institution  of  Civil  Engineers. 


REPRINTED  FROM   VAN  NOSTRAND'S   MAGAZINE. 


NEW    YORK: 

D.  VAN  NOSTRAND,  PUBLISHER, 

23  MURRAY  AND  27  WARREN  STREET. 

1881. 


OF  THE 

UNIVERSITY 


PREFACE 


THE  much  discussed  subject  of  the 
pressure  of  earthwork  is  in  this  essay  so 
exhaustively  treated  that  nothing  is  left 
to  be  desired.  The  engineer  may  find 
here  a  satisfactory  explanation  of  the 
causes  of  the  discrepancies  between 
theory  and  practice,  and  of  the  differ- 
ences between  different  authorities. 

It  was  originally  presented  as  a  paper 
to  the  Institution  of  Civil  Engineers, 
from  the  published  minutes  of  which  the 
essay  as  here  presented  was  published  in 
VAN  NOSTJRAND'S  MAGAZINE.  Abstracts 
only  of  the  discussion  are  presented 
here. 


OF  THE 

UNIVERSITT 


The   Actual    Lateral    Pressure 
of  Earthwork. 


THE  fact  that  a  mass  of  earthwork 
tends  to  assume  a  definite  slope,  and  that 
if  this  tendency  be  resisted  by  a  wall  or 
any  other  retaining  structure,  a  lateral 
pressure  of  notable  severity  will  be  ex- 
erted by  the  earthwork  on  that  structure, 
must  have  enforced  itself  upon  the  at- 
tention of  constructors  in  the  earliest 
ages.  Many  of  the  rudest  fortresses 
doubtless  had  revetments,  and  of  the 
hundreds  of  topes,  or  sacred  mounds, 
raised  in  India  and  Afghanistan  two 
thousand  years  ago,  not  a  few  afford  ex- 
amples of  surcharged  retaining  walls  on 
as  large  a  scale  as  those  occurring  in 
modern  railway  practice.  Nevertheless, 
long  as  the  subject  has  occupied  the  at- 
tention of  constructors,  there  is  proba- 
bly none  other  regarding  which  there  ex- 
ists the  same  lack  of  exact  experimental 


data,  and  the  same  apparent  indifference 
as  to  supplying  this  want.     Thousands 
of  pieces  of  wood  have  been  broken  in 
all  parts  of  the  world  to  determine  the 
transverse  strength  of  timber,  whilst  the 
experiments  that  have  been  undertaken 
to  ascertain  the  actual  lateral  pressure  of 
Earthwork  are  hardly  worth  enumerating* 
One  authority  after  another  has  simply 
evaded  the  task  of  experimental  investi- 
gation, by  assuming   that    some    of  the 
elements  affecting  the  stability  of  earth- 
work are  so  uncertain  in  their  operation 
as  to  justify  their  rejection,  and  have  so 
relieved  themselves  from  further  trouble. 
It  would  hardly  be  less  logical  to  assume 
that  because  timber  is  liable  to  become 
rotten  and  possesses  no  strength  at  all,  it 
was   therefore    unnecessary   to  conduct 
experiments  in  that  case  also.     As  a  mat- 
ter of  fact,  although  these  uncertain  ele- 
ments are   neglected   in   investigations, 
engineers  in   designing,  and   still  more 
contractors  in  executing,  works,  do  not 
neglect  them,  nor  could  they  do  so  with- 
out leading  to  a  blameworthy  waste  of 


OP  THE 


money  in  some  instances,  and  to  a  dis- 
creditable failure  in  others.  The  result 
of  the  present  want  of  experimental  data 
is  then  simply  that  individual  judgment 
has  to  be  exercised  in  each  instance,  with- 
out that  aid  from  careful  experimental 
investigation  which  in  these  times  is  en- 
joyed in  almost  every  other  branch  of 
engineering. 

The  mass  of  existent  literature  on  the 
subject  is  both  misleading  and  disap- 
pointing, for  with  little  exception  the 
bulk  of  it  consists  merely  of  arithmeti- 
cal changes  rung  upon  a  century-old 
theory,  which  even  at  the  time  of  its  in- 
ception was  put  forward  but  as  a  provi- 
sional approximation  of  the  truth,  pend- 
ing the  acquirement  of  the  necessary 
data.  Writing  some  fifty  years  ago  Pro- 
fessor Barlow  excused  his  u  very  imper- 
fect sketch  of  the  theory  of  revetments,  . 
at  least  as  relates  to  its  practical  applica- 
tion," on  the  ground  that  there  was  a 
"  want  of  the  proper  experimental  data  ;" 
and  but  comparatively  the  other  day  Pro- 
fessor Eankine  had  to  write  in  almost 


identical  terms :  "  There  is  a  mathematical 
theory  of  the  combined  action  of  fric- 
tion and  adhesion  in  earth ;  but  for  want 
of  precise  experimental  data  its  practi- 
cal utility  is  doubtful."  It  is  not,  there- 
fore, for  want  of  asking  that  the  missing 
data  are  not  forthcoming.  Indeed,  the 
present  desiderata  could  not  have  been 
more  clearly  formulated  than  they  were 
half  a  century  ago  by  Professor  Barlow 
in  the  following  words :  "  To  render  the 
theory  complete,  with  respect  to  its 
practical  application,  it  is  necessary  to 
institute  a  course  of  experiments  upon  a 
large  scale ;  upon  the  force  with  which 
different  soils  tend  to  slide  down  when 
erected  into  the  form  of  banks.  A 
well-conducted  set  of  experiments  of 
this  kind  would  blend  into  one  what 
many  writers  have  divided  into  sev- 
eral distinct  data.  %Thus  some  authors 
have  considered  first,  what  they  call 
the  natural  slope  of  different  soils,  by 
which  they  mean  the  slope  that  the 
surface  will  assume  when  thrown  loosely 
in  a  heap;  very  different,  as  they  sup- 


9 

pose,  from  the  slope  that  a  bank  will  as- 
sume that  has  been  supported,  but  of 
which  that  support  has  been  removed  or 
overthrown.  This,  therefore,  leads  to 
the  consideration  of  the  friction  and 
cohesion  of  soils,  and  what  is  denomina- 
ted the  slope  of  maximum  thrust ;  but, 
however  well  this  may  answer  the  pur- 
pose of  making  a  display  of  analytical 
transformations,  I  cannot  think  it  is  at 
all  calculated  to  obtain  any  useful  prac- 
tical results.  I  should  conceive  that  a 
set  of  experiments,  made  upon  the  abso- 
lute thrust  of  different  soils,  which  would 
include  or  blend  all  these  data  in  one 
general  result,  would  be  much  more  use- 
ful, as  furnishing  less  causes  of  error, 
and  rendering  the  dependent  computa- 
tions much  more  simple  and  intelligible 
to  those  who  are  commonly  interested  in 
such  deductions." 

A  knowledge,  however  imperfect,  of 
the  actual  lateral  pressure  of  earthwork, 
as  distinguished  from  what  may  be 
termed  the  "  text-book "  pressures, 
which,  with  hardly  an  exception  known 


10 

to  the  author,  are  based  upon  calcula- 
tions that  disregard  the  most  vital  ele- 
ments existent  in  fact,  is  of  the  utmost 
importance  to  the  engineer  and  con- 
tractor. It  affects  not  merely  the  stabil- 
ity of  retaining  walls,  but  the  strength 
of  tunnel  linings,  the  timbering  of  shafts, 
headings,  tunnels,  deep  trenches  for  re- 
taining walls,  and  many  other  works  of 
every-day  practice.  The  vast  divergence 
between  fact  and  theory  has  perhaps  im- 
pressed itself  with  peculiar  force  upon 
the  author,  because,  having  had  the 
privilege  of  being  associated  with  Mr. 
Fowler,  Past  President  of  thelnst.  C.E., 
during  the  whole  period  of  the  construc- 
tion of  the  "  underground "  system  of 
railways,  he  has  had  the  advantage  of 
the  experience  gained  in  constructing 
about  9  miles  of  retaining  walls,  and,  in 
relation  to  the  subject  of  the  present 
paper,  the  still  more  valuable  experience 
of  34  miles  of  deep-timbered  trenches 
for  retaining  walls,  sewers,  covered  ways, 
and  other  structures.  A  timber  waling 
is  a  sort  of  spring,  rough  it  may  be,  but 


11 

still  the  deflections  when  taken  over  a 
sufficiently  large  number  of  walings 
afford  an  approximate  indication  of  the 
pressure  sustained — an  advantage  which 
a  retaining  wall  does  not  possess.  Again, 
though  numberless  retaining  walls  have 
failed,  in  ninety-nine  cases  out  of  hun- 
dred the  failures  have  been  due  to  faulty 
foundations,  and,  consequently,  experi- 
ences  of  this  sort  seldom  afford  any  di- 
rect evidence  as  to  the  actual  lateral 
pressure  of  earthwork.  In  timbered 
trenches,  on  the  other  hand,  the  element 
of  sinking  and  sliding  foundations  does 
not  so  frequently  arise  to  complicate  the 
investigation. 

All  kinds  of  earth  were  traversed  by 
the  above  34  miles  of  trenches,  from 
light  vegetable  refuse  to  the  semi-fluid 
yellow  clay,  which  at  different  times  has 
crushed  in  so  many  tunnel  linings  in  the 
northern  districts  of  the  metropolis. 
The  heights  of  the  retaining  walls  ranged 
up  to  45  feet,  the  depths  of  the  timbered 
trenches  to  54  feet,  and  the  ground  at 
the  back  of  the  former  was  in  many 


12 

cases  loaded  with  buildings  ranging  up 
to  80  feet  in  height.  Possibly  some  of 
the  author's  observations  and  conclusions 
in  connection  with  these  and  other  works 
of  a  similar  character  may  be  of  interest 
to  engineers,  though  the  information  he 
is  able  to  contribute,  having  been  ob- 
tained chiefly  in  the  ordinary  routine  of 
his  practice  and  not  in  specially  devised 
investigations,  must  necessarily  form  but 
a  very  imperfect  contribution  to  the  data 
which  have  been  asked  for  so  long. 

.  The*  theory  underlying  all  the  multi- 
tudinous published  tables  of  required 
thickness  for  retaining  walls  is,  that  the 
lateral  pressure  exerted  by  a  bank  of 
earth  with  a  horizontal  top  is  simply  that 
due  to  the  wedge-shaped  mass,  included 
between  the  vertical  back  of  the  wall  and 
a  line  bisecting  the  angle  between  the 
vertical  and  the  slope  of  repose  of  the 
material.  If  this  were  true  in  practice, 
all  such  problems  could  be  solved  by 
merely  drawing  a  line  on  the  annexed 
diagram,  in  which  a  b  d  c  is  a  square,  a  b 
g  a  triangle,  having  the  sides  of  the 


13 


ratio  of  1 :    A/J,  and  a  h  d  a  parabolic 
curve.* 


--70 


*  For  earthwork  and  masonry  of  the  same  weight 
per  cubic  foot  the  equation  for  stability  is : 

Tit*     h*  4 
~-=-j-taai  }  angle. 

Hence,  the  required  thickness  (t\)  in  terms  of  the 
height  (K)  will  be  t\  —  ^^  tan  y2  angle,  which  is  repre- 
sented on  the  diagram  by  the  line  a— g ;  and  the 
"equivalent  fluid  pressure  "  in  terms  of  that  of  a  cu- 
bic foot  earthwork  will  be = tan  a  %  angle,  which  is 
represented  by  the  parabolic  curve  a  h  d. 


OF  THE 

XJNIVERSIT1T 


14 

Thus,  if  it  were  required  to  know  the 
lateral  pressure  per  square  foot  of  earth- 
work, having  a  slope  of  repose  of  1^: 
1,  and  the  thickness  of  rectangular  verti- 
cal wall  which,  when  turning  over  on  its 
outside  edge  would  just  balance  that 
pressure,  it  would  merely  be  necessary 
to  draw  the  line  c  f  at  the  given  slope  of 
1£ :  1  and  the  line  c  e  bisecting  the  angle 
a  c  k,  when  the  line  e  h  would  give  the 
equivalent  fluid  pressure  in  terms  of  that 
of  a  cubic  foot  of  the  earth =28. 7  per 
cent.,  and  the  line  e  i  the  thickness  of 
the  rectangular  wall  in  terms  of  the 
height =31  per  cent ,  the  weight  of  ma- 
sonry being  the  same  as  that  of  the 
earth. 

Common  stocks  in  mortar  and  ballast 
backing  each  weigh  about  100  Ibs.  per 
cubic  foot,  hence,  on  the  preceding  hy- 
pothesis, the  pressure  acting  on  the  wall 
would  be  the  same  as  that  due  to  a  fluid 
weighing  28.7  Ibs.  per  cubic  foot.  If,  as 
is  usually  the  case,  the  masonry  be 
heavier  than  the  earthwork,  the  required 
thickness  of  wall  would  be  reduced  in 


15 


inverse  proportion  to  the  square  root 
of  the  respective  weights,  so  that  should 
the  masonry  weigh  10  per  cent,  more 
than  the  ballast,  the  thickness  would  be 
about  5  per  cent,  less  than  before,  or«> 
say,  29.5  per  cent,  of  the  height. 

For  other  slopes  of  repose  the  equiva- 
lent fluid  pressure  and  thickness  of  wall 
for  materials  of  equal  weight  would  be 
as  follows : 


Ratio  of  horiz-  )       ~ 
ontal  to  vertical  f 

.6 

.7 

.8 

.9 

1.0 

Fluid  pressure.  .  .  5.6 

7.7 

10 

12.4 

14.8 

17.2 

Thickness     .            136 

160 

.182 

903 

222 

939 

Ratio  of  horiz-  [ 
ontal  to  vertical  ) 

1.1 

1.2 

1.3 

1.4 

1.5 

1.6 

Fluid  pressure.  .. 

19.6 

22 

24.3 

26.5 

28.7 

30.7 

Thickness       .... 

956 

M 

984 

997 

31 

39, 

16 


Ratio  of  horiz-  ) 
ontal  to  vertical  f 

Fluid  pressure  .  .  . 

1.7 

1.8 

2 

3 

52 

4 
61 

00 

100 

32.8 

34.6 

38.2 

Thickness  

.33 

.34 

.357 

.416 

.451 

.578 

In  the  thickness  tabulated  above  no 
allowance  has  been  made  for  the  crush- 
ing action  on  the  outer  edge ;  in  practice 
the  batter  usually  given  to  the  face  of 
the  wall  more  than  compensates  for  this 
action  if  the  mean  thickness  be  that 
given  in  the  table.  No  factor  of  safety 
is  included,  but  according  to  theory  the 
wall  in  each  case  would 'be  just  on  the 
balance.  Any  one  accustomed  to  deal 
with  works  of  this  class  will,  however, 
know  that  in  practice  walls  so  propor- 
tioned would  in  the  majority  of  cases 
possess  a  large  factor  of  safety. 

Doubtless  many  engineers  will,  with 
the  author,  have  noticed  that  laborers 
and  others  not  infrequently  carry  out 
unconsciously  a  number  of  valuable  and 


17 

suggestive  experiments  on  The  Actual 
Lateral  Pressure  of  Earthwork.  In 
stacking  materials,  rough  and-ready  re- 
taining walls,  made  of  loose  blocks  of 
the  same  material,  are  often  run  up,  and 
as  it  is  generally  of  little  moment 
whether  a  slip  occurs  or  not,  the  work- 
men do  not  trouble  about  factors  of 
safety,  but  expend  the  least  amount  of 
labor  that  their  every-day  experience  will 
justify,  and  so  a  tolerably  close  measure 
is  obtained  of  the  average  actual  press- 
ure of  material  retained.  When  the 
wood  paving  was  recently  laid  in  Eegent 
Street,  the  space  being  limited,  the 
stacked  wooden  blocks  in  many  cases 
had  to  do  duty  as  retaining  walls  to  hold 
up  the  broken  stone  ballast  required  for 
the  concrete  substructure.  In  one  in- 
stance (Ex.  1)  the  author  noted  that  a 
wall  of  pitch-pine  blocks,  4  feet  high  and 
1  foot  thick,  sustained  the  vertical  facef 
of  a  bank  of  old  macadam  materials  which 
had  been  broken  up,  screened,  and  tossed 
against  this  wall  until  the  bank  had  at- 
tained a  height  of  3  feet  9  inches,  a 


18 

width  at  the  top  of  about  5  feet,  and 
slopes  on  the  farther  sides  deviating 
little  from  1.2  to  1.  Now,  referring  to 
the  diagram  and  table  of  thickness,  it 
will  be  seen  that  according  to  the  ordi- 
nary theory  the  thickness  of  wall  which 
would  just  balance  the  thrust  of  a  bank 
3  feet  9  inches  high  of  material  haying  a 
slope  of  repose  of  1.2  to  1  would  be 
3.75 X. 27=  1.01,  or,  say,  1  foot,  which  is 
the  actual  thickness  of  the  given  wall. 
But  in  the  table  the  specific  weight  of 
the  material  in  the  wall  and  backing  is 
assumed  to  be  the  same,  whereas  in  the 
present  case  the  weight  of  the  pitch-pine 
block  wall,  allowing  for  the  height  being 
greater  than  that  of  the  bank,  would 

4  feet 
only  be,  say,  46  Ibs.  X  »  „     .      =±3  Ibs. 

per  cubic  foot,  whilst  that  of  the  broken 
granite  bank  would  be,  say,  168  Ibs.  less 
40  per  cent,  for  interstices  =  101  Ibs.  per 
cubic  foot.  It  follows,  since  the  wooden 
wall  stood,  that  if  it  had  been  made  of 
materials  having  the  same  weight  per 
cubic  foot  as  the  bank,  the  retaining  wall 


OP  THE 


19 


would  not  have  been  on  the  point  of 
toppling  over,  as  the  ordinary  theory 
would  indicate,  but  have  possessed  a 

factor  of  safety  of  at  least  —  -  ,  or, 

4y  lus. 

say,  2  to  1.  The  effective  lateral  press- 
ure of  the  earthwork  in  this  instance 
consequently  could  not  have  exceeded  a 

fluid  pressure  of  -  -  =  10.7   Ibs. 

per  cubic  foot,  instead  of  the  22  Ibs., 
which  theoretically  corresponds  to  the 
given  slope  of  1.2  to  1. 

Taking  another  case,  in  which  the 
wall,  instead  of  being  lighter  than  the 
bank,  was  much  heavier,  the  same  con- 
clusion still  holds  good.  In  this  instance 
(Ex.  2)  the  author  found  a  wall  of  slag 
blocks  having  a  batter  of  \  of  the  height, 
and  an  effective  thickness  of  1  foot  sus- 
tained a  bank  of  broken  slag  10  feet 
high,  with  a  surcharge  of  some  5  feet 
more.  The  battering  wall,  with  a  thick- 
ness of  ^  of  the  height,  would  have  the 
same  stability  as  a  vertical  wall  0.173 
thick,  and  the  lateral  pressure  of  the  sur- 


20 

charged  bank  with  the  battering  face 
would  be  practically  the  same  as  that  of 
a  horizontal-topped  bank  with  a  vertical 
face;  hence,  since  the  relatively  closely- 
packed  slag  blocks  constituting  the  wall 
would  weigh  about  40  per  cent,  more 
than  the  broken  slag  of  the  bank,  the 
thickness  of  a  vertical  wall  built  of  ma- 
terials of  the  same  weight  as  the  bank, 
and  having  the  same  stability  as  the  wall 
under  consideration  would  be  =  Vl  •  4  X 
0.173  =  0.205  of  the  height.  Referring 
to  the  table,  the  figure  0.205  will  be 
found  to  apply  to  a  slope  of  repose  of 
0.8  to  1,  whereas  the  actual  slope  in  the 
instance  of  this  slag  was  1.33  to  1.  For 
the  latter  slope  the  thickness  theoret- 
ically should  have  been  0.29,  and  since 
the  stability  varies  as  the  square  of  the 
thickness,  it  follows  that  with  the  thick- 
ness indicated  by  theory,  the  wall,  in- 
stead of  being  just  on  the  balance,  would 
have  possessed  a  factor  of  safety  of  at 

0.29'" 

least  —  ^— 2,  or   2   to   1,  as   in    the   last 
O.^Oo 

example. 


21 

Other  instances  of  these  unintentional 
experiments  on  the  lateral  pressure  of 
earthwork  will  be  found  in  the  stacking 
of  coal  in  station  yards,  in  the  rubbish 
banks  at  quarries,  and  in  many  other 
instances  which  have  been  investigated 
by  the  author,  with  the  invariable  result 
of  finding  that  walls  which,  according  to 
current  theory,  would  be  on  the  point  of 
failure,  really  possess  a  considerable 
factor  of  safety. 

Turning  now  from  indirect  to  direct 
experiments,  specially  arranged  with  a 
view  to  determine  the  lateral  pressure  of 
earthwork,  those  carried  out  at  Chatham 
nearly  forty  years  ago  by  Lieutenant 
Hope,  K.E.,  may  be  referred  to.  His 
intention  was  to  experiment  first  with 
fine  dry  sand,  as  free  as  possible  from 
the  complications  introduced  by  cohesion, 
irregularities  of  mass  and  other  practical 
conditions,  and  then  to  extend  the  in-* 
vestigation  to  ordinary  shingle,  and  to 
clay  and  other  soils  possessed  of  great 
tenacity.  Sand  and  shingle  were,  how- 
ever, alone  experimented  with. 


22 

The  direct  lateral  thrust  of  sand 
weighing  91  Ibs.  per  cubic  foot  when 
lightly  thrown  together,  and  98 J  Ibs. 
when  well  shaken,  was  measured  by 
balancing  the  pressure  exerted  on  a 
board  1  foot  square.  The  mean  results 
of  seven  experiments  (Ex.  3)  was  9  Ibs. 
7  oz.,  which  is  that  due  to  a  fluid  weigh- 
ing nearly  19  Ibs.  per  cubic  foot.  As 
the  slope  of  repose  of  the  sand  employed 
was  1.42  to  1,  the  theoretical  fluid  press- 
ure due  to  the  weight  of  98J  Ibs.  per 
cubic  foot  wou]d  be  26.2  Ibs.,  or  about 
40  per  cent,  more  than  the  observed  19 
Ibs.  per  cubic  foot. 

With  gravel  (Ex.  4)  weighing  95J  Ibs. 
per  cubic  foot,  and  having  a  slope  of  re- 
pose of  1J  to  1,  about  the  same  lateral 
pressure  was  found  to  exist.  Lieutenant 
Hope  attempted  to  reconcile  the  differ- 
ence between  theoretical  and  actual  re- 
sults by  adding  to  the  measured  force  an 
estimated  sum  for  friction  against  the 
sides  of  the  apparatus,  but  experiments 
of  the  author's  to  be  subsequently  refer- 
red to,  clearly  prove  that  the  difference 


23 


is  not  to  be  so  accounted  for.  Indeed, 
the  knowledge  of  what  the  pressure  theo- 
retically should  be  would  appear  to  have 
given  Lieutenant  Hope  an  unconscious 
bias  in  the  direction  of  rather  exaggerat- 
ing the  experimental  results.  This  it  is 
extremely  easy  to  do,  as  a  trifling  amount 
of  vibration  will  alter  the  pressure  from 
10  to  50  per  cent.,  and  a  comparatively 
innocent  shake  in  a  small  model  will  cor- 
respond in  its  relative  effects  with  an 
earthquake  in  real  life. 

Experiments  with  colored  sand  in  a 
vessel  with  glass  sides  did  not  uniformly 
confirm  the  usual  theory  that  the  angle 
of  pressure  of  maximum  thrust  is  half 
that  contained  between  the  natural  slope 
and  the  back  of  the  wall  (Ex.  5).  Thus 
the  line  of  separation  was  at  an  angle  of 
24°  with  the  vertical  instead  of  28°. 
Again,  with  a  gravel  bank  (Ex.  6)  10  feet 
high  the  line  of  separation  ranged  from  ' 
3  feet  8  inches  to  5  feet  8  inches  from 
the  back  of  the  wall,  whilst  as  the  natural 
slope  was  1J  to  1,  the  distance  should 
have  been  5  feet  in  all  instances  if  Cou- 


24 

lomb's  theory  applied  strictly  to  even 
such  exceptionally  favorable  materials  as 
dry  sand  and  shingle. 

The  really  valuable  portion  of  Lieu- 
tenant Hope's  investigation  was  the 
series  of  experiments  on  walls  built  of 
bricks  laid  in  wet  sand.  The  first  of 
these  (Ex.  7)  was  about  20  feet  long  and 
two-and-a-half  bricks,  or  say,  1  foot  11 
inches  thick.  When  raised  to  a  height  of 
8  feet  and  backed  with  ballast,  it  had  in- 
clined from  the  vertical  about  1-J  inch; 
at  9  feet  the  inclination  had  increased  to 
3J  inches,  and  at  10  feet  the  wall  fell  for- 
ward in  one  mass.  At  the  instant  when 
the  thrust  of  the  ballast  overcame  the 
stability  of  the  wall,  the  overhang  must 
have  been  4  inches,  and  the  moment  of 
stability  per  lineal  foot  certainly  not 
more  than  2,000  Ibs.x0.9  foot=l,800 

foot-pounds.     Hence,  dividing   by          , 

is  obtained  10.8  Ibs.  per  cubic  foot  as  the 
weight  of  the  fluid,  which  would  have 
exerted  a  lateral  pressure  equal  to  that 
of  the  ballast  piled  against  this  10-feet 


25 

wall.  This  is  hardly  more  than  half  the 
pressure  obtained  with  the  1-foot  square 
board,  and  shows  how  desirable  it  is 
that  even  the  most  faithful  experimenter 
should  not  know  what  to  expect  if  a 
mere  shake  of  a  table  will  enable  him  to 
obtain  the  desired  result.  The  natural 
slope  of  the  ballast  being  1J  to  1,  and 
the  weight  95 ^  Ibs.  per  cubic  foot,  the 
pressure  theoretically  should  have  been 
23.6  Ibs.  per  cubic  foot  instead  of  10.8 
Ibs  ;  hence  a  wall  so  proportioned  as  to 
be  on  the  point  of  toppling  over,  accord- 
ing to  the  ordinary  theory,  would  in  this 
instance  have  had  a  factor  of  safety  of 
rather  more  than  2  to  1. 

Another  vertical  wall  (Ex.  8)  was  con- 
structed with  the  same  amount  of  ma- 
terials differently  disposed.  At  8  feet 
high,  after  heavy  rain,  the  18-inch  thick 
panel  between  the  27-inch  deep  counter- 
forts had  bulged  1J  inch;  at  12  feet  10 
inches  the  bulging  had  increased  to  4 J 
inches,  and  the  overhang  at  the  top  to 
7£  inches,  when,  after  some  hours'  grad- 
ual movement,  the  wall  fell.  The  moment 


26 


of  stability  at  the  time  of  failure  could 
not  have  exceeded  2,600  Ibs.  X  1  foot  = 
2,600  foot  pounds,  which,  divided  by 

A8 

— ,  gives  7.4  Ibs.  per  cubic  foot,  instead 

of  the  theoretical  23.6  Ibs.,  as  the  weight 
of  the  equivalent  fluid.  This  result  is 
clearly  not  evidence  that  the  pressure  of 
the  ballast  was  less  in  the  counterforted 
wall  than  in  the  wall  of  uniform  thick- 
ness, but  that  the  binding  of  the  ballast 
between  the  counterforts  increased  the 
stability  of  the  wall  by  practically  add- 
ing somewhat  to  its  weight. 

A  wall  with  a  batter  of  ^  of  the  height, 
and  with  counterforts  of  the  same  thick- 
ness as  the  last  (Ex.  9),  was  next  tried, 
with  noteworthy  results.  This  wall, 
only  18  inches  thick,  with  counterforts  3 
feet  9  inches  deep,  measuring  from  the 
face  of  the  wall,  and  10  feet  apart,  was 
carried  to  a  height  of  21  feet  6  inches 
without  any  indications  of  movement, 
beyond  a  bulging  about  halfway  up  of  2^ 
inches  at  the  panel,  and  1J  inch  at  the 
counterfort;  and  in  Lieutenant  Hope's 


27 

opinion  it  would  probably  have  stood  for 
years  without  giving  way  any  more,  al- 
though the  mean  thickness  was  less  than 
^  of  the  height.  The  calculated  stabil- 
ity indicates  that  a  fluid  pressure  of  8.5 
Ibs.  per  cubic  foot  would  have  overturned 
the  wall,  and,  correcting  for  the  reduced 
thrust  of  the  ballast  due  to  the  batter  of 
its  face,  the  equivalent  pressure  on  a 
vertical  wall  would  be  that  of  a  fluid 
weighing  10  Ibs.  per  cubic  foot. 

Here,  again,  doubtless  the  binding  of 
the  gravel  between  the  counterforts  con- 
tributed to  the  stability  of  the  wall ;  but, 
even  adopting  the  extreme  and  impossi- 
ble hypothesis  that  the  ballast  was  as 
good  as  so  much  brickwork,  or,  in  other 
words,  that  the  wall  was  a  monolithic 
structure  of  the  uniform  thickness  of  3 
feet  9  inches,  its  stability  would  barely 
balance  the  23.6  Ibs.  per  cubic  foot 
fluid  pressure  theoretically  due  to  the 
weight  and  slope  of  repose  of  the  back- 
ing. Assuming  that  the  binding  of  the 
ballast  between  the  counterforts  in- 
creased the  stability,  as  in  Examples  8  and 


28 

9,  by  about  45  per  cent.,  the  fluid  resist- 
ance would  be  14.5  Ibs.  per  cubic  foot ; 
and,  remembering  that  this  wall  did  not 
fall,  though  the  bricks  were  only  laid  in 
sand,  it  is  reasonable  to  infer  that  this 
interesting  experiment  confirms  the  pre- 
vious conclusion  that  a  properly  built 
wall  in  mortar  or  cement,  just  balancing 
the  theoretical  pressure,  would  really 
have  had  a  factor  of  safety  of  2  to  1. 
Other  experiments  of  Lieutentant  Hope's 
justify  this  inference,  and  so  do  the  ex- 
periments of  General  Pasley,  also  made 
at  Chatham  many  years  ago. 

General  Pasley  experimented  with 
loose  dry  shingle  weighing  89  Ibs.  per 
cubic  foot,  and  having  a  natural  slope  of 
1J  to  1.  His  model  retaining  walls  (Ex. 
10)  were  3  feet  long,  26  inches  high,  of 
various  forms  and  thickness,  and  weighed 
84  Ibs.  per  cubic  foot.  The  stability  of 
each  wall  was  tried  by  pulling  it  over  by 
weights  before  and  after  backing  it  up 
with  shingle,  and  the  difference  between 
the  two  pulls  of  course  represented  the 
thrust  of  the  shingle.  When  the  thick- 


29 

ness  of  the  vertical  wall  was  8  inches, 
the  stability,  without  shingle,  was  equiv- 
alent to  a  pull  of  47  Ibs.  applied  at  the 
top  of  the  wall,  and  with  shingle,  the 
pull  required  to  upset  it  was  reduced  to 
30  Ibs.  The  difference  of  17  Ibs.  repre- 
sents the  thrust  of  the  shingle,  and 
throughout  the  several  hundreds  of  ex- 
periments this  appears  to  have  been  com- 
prised within  the  limits  of  16  Ibs.  and  24 
Ibs.  The  center  of  pressure  being  at  J 
of  the  height  of  the  wall,  the  mean 
thrust  of  20  Ibs.  at  the  top  will  be  equiv- 
alent to  60  Ibs.  at  the  center  of  pressure, 
and  the  area  being  6.5  square  feet,  and 
the  height  26  inches,  the  actual  lateral 
pressure  of  the  shingle,  as  deduced  from 
General  Pasley's  experiments,  is  equiva- 
lent to  that  of  a  fluid  weighing  8.5  Ibs. 
per  cubic  foot,  instead  of  21  Ibs.  as 
theory  would  indicate. 

General  Cunningham  tested  some 
model  revetments,  and  his  experiments 
led  him  to  believe  that  General  Pasley 
had  overestimated  the  thickness  required 
for  stability.  The 


30 

about  30  inches  in  height,  were  weighted 
with  earth  and  musket  bullets  to  the 
equivalent  of  an  equal  mass  of  masonry 
weighing  129  Ibs.  per  cubic  foot.  One 
of  the  models  (Ex.  11)  represented  a 
wall  30  feet  high,  6  feet  thick  at  the  base, 
vertical  at  the  back,  battering  1  in  10  on 
the  face,  with  counterforts  4  feet  3  inches 
thick,  18  feet  from  center  to  center,  and 
of  a  depth  equal  to  the  thickness  of  the 
wall  or,  say,  3  feet  at  the  top  and  6  feet 
at  the  base.  This  was  backed  up  and 
surcharged  with  shingle  weighing  104 
Ibs.  per  cubic  foot,  but  required  a  pull 
of  111  Ibs.  to  overturn  it.  Another 
model  (Ex.  12)  representing  a  wall  18 
feet  high,  4  feet  4  inches  thick  at  the 
base,  and  2  feet  8  inches  thick  at  the 
coping,  without  counterforts,  when  sur- 
charged with  shingle  to  a  height  great- 
er than  that  of  the  wall,  required  a 
pull  of  84  Ibs.  to  upset  it.  A  fluid  press- 
ure of  19  Ibs.  per  cubic  foot  would  over- 
come the  stability  of  such  a  wall ;  hence, 
having  regard  to  the  surcharge  and  to 
the  pull,  it  will  be  found  that  the  actual 


31 

lateral  pressure  of  the  shingle  could  not 
have  exceeded  that  due  to  a  fluid  weigh- 
ing 8  Ibs.  per  cubic  foot. 

General  Burgoyne  also  commenced  an 
experimental  investigation  of  the  ques- 
tion of  retaining  walls,  but  circumstances 
precluded  his  pursuing  the  subject. 
About  half  a  century  ago  he  built  at 
Kingstown  four  experimental  walls  20 
feet  long  and  20  feet  high,  having  the 
same  mean  thickness  of  3  feet  4  inches, 
or  ^  of  the  height,  but  differing  otherwise. 
One  of  them  (Ex.  13)  was  of  the  uniform 
thickness  of  3  feet  4  inches,  and  battered 
-^  of  the  height ;  another  (Ex.  14)  was  1 
foot  4  inches  thick  at  the  top,  and  5  feet 
4  inches  at  the  bottom,  with  a  vertical 
back;  the  third  (Ex.  15,  Fig.  1;  was  of 
the  same  dimensions,  with  a  vertical 
front ;  and  the  last  (Ex.  16,  Fig.  2)  was 
a  plain  rectangular  vertical  wall  3  feet  4 
inches  thick.  The  masonry  consisted 
simply  of  rough  granite  blocks  laid  dry, 
and  the  filling  was  of  loose  earth  filled 
in  at  random,  without  ramming  or  other 
precautions,  during  a  very  wet  winter. 


32 

No.  1  wall  stood  perfectly,  as  might  have 
been  expected  from  the  behavior  of  Lieu- 
tenant Hope's  experimental  wall  of  near- 
ly the  same  height  and  batter.  No.  2 
wall  also  stood  well,  coming  over  only 


Fig.1 


Fig-2 


abont  2  inches  at  the  top.  A  fluid  press- 
ure of  22.5  Ibs.  per  cubic  foot  would  be 
required  to  overcome  the  stability  of  this 
dry  masonry  wall  weighing  142  Ibs.  per 
cubic  foot.  Earthwork  of  the  class  de- 


33 


scribed,  consolidated  during  continuous 
rain,  would  not  weigh  less  than  112  Ibs. 
per  cubic  foot,  nor  have  a  slope  of  re- 
pose less  than  1£  to  1.  Referring  to  the 
table,  the  theoretical  pressure  of  such 
earthwork  would  be  28.67x1.12  =  32  Ibs. 
per  cubic  foot,  or  nearly  one-half  greater 
than  the  wall  could  resist. 

No.  3  and  No.  4  walls  both  fell  when 
the  filling  had  attained  a  height  of  17 
feet.  The  former  came  over  10  inches  at 
the*top,  was  greatly  convex  on  the  face, 
overhanging  5  inches  in  the  first  5  feet  of 
its  height  and  rending  it  in  every  direc- 
tion, when  finally  it  burst  out  at  5  feet  6 
inches  from  the  base,  and  about  two- 
thirds  of  the  upper  portion  of  the  wall 
descended  vertically  until  it  reached  and 
crushed  into  the  ground  (Fig.  1).  The 
vertical  wall  tilted  over  gradually  to  18 
inches  and  then  broke  across,  as  it  were,  • 
at  about  ^  of  its  height  and  fell  forward 
(Fig.  2).  So  long  as  the  wall  remained 
vertical  the  calculated  stability  would  in- 
dicate it  to  be  equal  to  sustain  the  press- 
ure of  a  fluid  weighing  20.4  Ibs.  per 


34 

cubic  foot,  but  the  overhang  of  18  inches 
and  the  bulging  which  occurred  would 
reduce  the  stability  exactly  one-half,  so 
that  a  fluid  pressure  of  10.2  Ibs.  would 
really  have  sufficed  to  effect  the  final 
overthrow.  The  character  of  the  failure 
both  of  No.  3  and  No.  4  walls  clearly  in- 
dicates that  if  the  walls  had  been  in 
mortar  or  cement,  as  usual,  the  overhang 
would  not  have  been  a  fraction  of  that 
occurring  with  the  dry  stone  walling, 
and  the  failure  would  not  have  faken 
place.  Since,  as  already  stated,  the  theo- 
retical thrust  of  the  earthwork  would  be 
3;21bs.  per  cubic  foot,  it  is  hardly  unfair 
to  conclude  that  a  wail  in  mortar  and  pro- 
portional to  that  pressure  would  not  have 
come  over  and  would  have  enjoyed  a  fac- 
tor of  safety  of  at  least  2  to  1. 

Colonel  Michon  carried  out  in  1863  an 
interesting  experiment  (Ex.  17)  on  a  40 
feet  high  retaining  wall  of  a  peculiar  type 
(Figs.  3  and  4),  which,  perhaps,  may  be 
best  described  as  a  very  thin  w&ll  with 
numerous  battering  buttresses  turned 
upside  down.  The  face  wall,  battering 


35 


1  in  20,  was  only  1  foot  8  inches  thick, 
and  the  buttresses,  spaced  about  5  feet 
apart  from  center  to  center,  were  also  1 


Fig.4 


foot  8  inches  thick  by  2  feet  4  inches 
deep  at  the  base  and  9  feet  2  inches  at 
the  top.  The  work  was  hurriedly  con- 


36 

s  true  ted  during  continuous  rains  with 
any  stones  that  came  to  hand,  and  with 
very  bad  lime.  When  the  filling  had 
attained  a  height  of  29  feet  the  wall 
bulged  a  trifle,  but  no  further  movement 
was  noticed,  though  the  filling,  when 
carried  up  to  the  top  of  the  coping, 
was  allowed  some  weeks  to  settle  in  the 
rain.  Earth  was  then  piled  above  the 
level  of  the  coping  to  a  height  of  be- 
tween 3  and  4  feet,  when  the  wall  fell. 
The  fall  was  preceded  by  a  general  dis- 
location of  the  masonry  at  the  base,  a 
bulging  at  about  one-third  of  the  height, 
and  a  slight  movement  of  the  top  to- 
wards the  bank.  The  lower  portion  of 
the  wall  fell  outwards,  the  upper  part 
dropped  vertically  (as  in  General  Bur- 
goyne's  wall,  Fig.  1),  and  a  considerable 
number  of  the  counterforts  went  for- 
ward with  the  slip  and  even  maintained 
their  vertical  position.  • 

This  failure  arose  from  a  flexure  of 
the  thin  wall  at  the  center  of  pressure  of 
the  earthwork,  and  would  not  have  oc- 
curred had  the  masonry  been  in  cement 


37 

instead  of  in  weak  unset  lime.  No  direct 
data  therefor  are  afforded  for  an  exact 
estimate  of  the  actual  lateral  thrust  of 
the  heavy  wet  filling  on  this  lofty  wall. 
Nevertheless,  as  the  weight  of  the  ma- 
sonry was  only  18,000  Ibs.  per  lineal 
foot,  and  the  center  of  gravity  of  the 
same  from  the  toe  but  6  feet  6  inches,  it 
follows  that  the  wall,  even  if  monolithic, 
would  be  overturned  with  the  pressure  of 
a  fluid  weighing  11  Ibs.  per  cubic  foot. 
How  far  the  sodden  earthwork  between 
the  counterforts  contributed  to  the  sta- 
bility of  the  wall  is  open  to  question, 
but  it  could  hardly  account  for  the  differ- 
ence between  the  11  Ibs.  or  less  stability 
and  the  32  Ibs.  due,  according  to  the  or- 
dinary theory,  to  the  weight  and  slope  of 
the  backing.  If  dirt  were  as  good  as 
masonry,  General  Burgoyne's  wall  with 
the  battering  back  (Fig.  1)  would  have 
been  more  stable  than  the  vertical  wall 
(Fig.  2)  in  the  ratio  of  the  squares  of 
their  respective  bases,  or,  say,  as  2^  tol, 
whereas,  these  walls  proved  to  be  of 
equal  stability,  both  falling  with  17  feet 


38 

of  filling.  Colonel  Michon,  by  assuming 
dirt  to  be  as  good  as  masonry,  and  a  wall 
40  feet  high  and  1  foot  8  inches  thick  of 
unselected  stones  and  unset  mortar  to  be 
as  good  as  a  monolith,  succeeds  in  recon- 
ciling the  behavior  of  his  wall  with  the 
ordinary  theory  of  the  stability  of  earth- 
work ;  but  in  the  author's  experience  the 
conditions  assumed  are  not  approached 
in  practice.  The  stability  of  this  lofty 
wall  battering  only  -fa  of  the  height  on 
the  face,  and  averaging  hardly  more  than 
T*2  of  the  height  in  thickness,  is,  never- 
theless, one  of  the  most  remarkable  and 
interesting  facts  connected  with  the  sub- 
ject of  the  present  paper. 

To  show  how  invariably  an  experi- 
mentalist is  driven  to  the  same  conclu- 
sion as  to  the  excess  in  the  theoretical 
estimate  of  the  pressure  of  earthwork, 
the  "  toy  "  experiments  of  Mr.  Casimer 
Constable  with  little  wooden  bricks  and 
peas  for  filling  may  be  usually  referred 
to.  The  peas  took  a  slope  of  1.9  to  1, 
and  weighed  twice  as  much  per  cubic 
foot  as  the  wooden  retaining  wall.  By 


39 

the  table  the  thickness  of    wall,   which 
would  just  balance  the  lateral  pressure 
would   be   .35\/2  =  .49   height.     By  ex- 
periment, a  wall  ( Ex,  18)  having  a  thick- 
ness of   .40  height  moved  over  slightly, 
but  took  some  amount  of  jarring  to  bring 
it  down.     Since   the   stability  varies   as 
the  square  of  the  thickness,  the  calcula- 
ted wall  would  be  50  per  cent,  more  sta- 
ble than  the  actual  wall,  without  consid- 
ering  the   question    of  jarring.     If  the 
slope  of  the  peas  had  been  measured  also, 
after  jarring,  it  would  probably  have  been 
found  to  be  nearer  2.9  to  1  than  1.9  to  1, 
and  the    calculated   required   thickness 
would   have    been    correspondingly    in- 
creased. 

The  influence  of  even  a  slight  amount 
of  vibration  is  well  illustrated  by  the 
difference  between  the  co-efficient  of 
friction  of  stones  on  one  another  in  mo- 
tion and  repose.  Granite  blocks,  which 
will  start  on  nothing  flatter  than  1.4  to  1, 
will  continue  in  motion  on  an  incline  of 
2.2.  to  1,  and,  for  similar  reasons,  earth- 
work  will  assume  a  flatter  slope  and  ex- 


40 


ert  a  greater  lateral  pressure  under  vi- 
bration than  when  at  rest.  This  fact  has 
long  received  practical  recognition  from 
engineers ;  indeed,  attention  was  called 
to  it  by  Mr.  Charles  Hutton  Gregory,  C. 
M.G.,  Past-President  Iiist.  C.E.,  in  a  Pa- 
per on  slips  in  earthwork,  read  before 
the  Institution  in  1844,  when  the  Presi- 
dent and  others  gave  instances  of  slips 
in  railway  cuttings  caused  by  vibration; 
The  general  results  of  the  preceding 
and  other  independent  experiments  on 
retaining  walls  tending  to  throw  a  doubt 
on  the  accuracy  of  Lieutenant  Hope's 
measurement  of  the  direct  lateral  thrust 
of  ballast  and  sand  on  a  board  1  foot 
square,  the  Author  considered  it  advisa- 
ble to  repeat  those  experiments.  Care 
was  taken  to  eliminate  all  disturbing 
causes  tending  to  vitiate  the  results.  The 
pressure  board  was  held  by  a  string  at 
its  center  of  pressure,  and  was  perfectly 
free  to  move  in  every  direction,  which,  of 
course,  a  retaining  wall  having  a  greater 
hold  on  the  ground  than  stability  to  re- 
sist overturning  has  not.  In  every  in- 


41 

stance  the  filling  was  poured  into  the  bo;< 
and  allowed  to  assume  its  natural  slope 
towards  the  pressure  board,  and  the  lat- 
ter was  rotated  and  thumped  to  keep  the 
ballast  alive  before  the  reaction  was  meas- 
ured. In  order  to  avoid  all  chance  of  the 
bias  which  the  knowledge  what  to  expect 
might  have  given  him,  as  it  did  Lieuten 
arit  Hope,  the  author  had  the  experiment8 
made  by  others  who  were  ignorant  even 
of  the  object  of  them,  whilst  he  himself 
purposely  experimented  with  an  appara- 
tus the  dimensions  of  which  he  did  not 
know,  and  consequently  could  form  no 
estimate  of  the  weight  which  would  be 
required  in  the  scale 

With  clean  dry  ballast  having  a  natu- 
ral slope  of  1 J  to  1,  it  will  be  remembered 
Lieutenant  Hope  obtained  a  lateral  press- 
ure on  1  square  foot  of  9  Ibs.  7  oz.  With 
well-washed  wet  ballast  of  the  same  kind 
the  author  found  the  natural  slope  to  be 
1£  to  1,  and  he  decided  therefore  to  use 
the  ballast  wet,  because,  possessing 
greater  fluidity,  it  would  give  more  uni- 
form results  than  dry  ballast,  and  also 


42 

impose  greater  lateral  pressure.  In  a, 
large  number  of  independent  experiments 
the  results  were  uniformly  as  follows 
(Ex.  19) :  With  6  Ibs.  in  the  scale  the 
board  moved  forward  about  ^  inch,  but 
continued  to  retain  the  ballast ;  with  7 
Ibs.  very  slight  movement  occurred ;  with 
8  Ibs.,  no  movement  at  all ;  and  with  10 
Ibs.,  under  extreme  vibration,  the  board 
moved  forward  about  as  much  as  it  did 
with  6  Ibs.  without  vibration.  The  gen- 
eral opinion  of  the  different  experiment 
alists  was,  that  the  fair  value  of  the  lat- 
eral pressure  of  this  wet  ballast  was  7 
Ibs.,  because  when  that  weight  was  in  the 
scale  pad  a  slight  jolt  was  sufficient  to 
let  the  ballast  down  by  the  run  to  a  slope 
of  1  £  to  1.  The  board  being  1  foot  square, 
this  of  course  is  equivalent  to  the  press- 
ure of  a  fluid  weighing  14  Ibs.  per  cubic 
foot,  instead  of  the  19  Ibs.  obtained  by 
Lieutenant  Hope,  and  the  26  Ibs.  indica- 
ted by  theory. 

With  the  same  ballast  unwashed  and 
mixed  with  slightly  loamy  pit  sand  (Ex. 
20)  the  natural  slope  was  1  to  1,  without 


43 

vibration,  and  1£  to  1  with  a  moderate 
amount  of  vibration.  A  weight  of  3  Ibs. 
was  as  effective  in  retaining  this  ballast 
as  6  Ibs.  in  -the  former  instance ;  4  Ibs, 
held  it  under  a  moderate  amount  of  vi- 
bration, but  3£  Ibs.  failed  to  hold  the 
board  under  very  little.  Practically 
speaking,  the  lateral  thrust  was  about 
half  that  with  the  clean  wet  ballast,  and 
considerably  less  than  half  that  theoreti- 
cally due  to  the  slope  of  repose  of  the 
loamy  ballast. 

In  harbor  works  both  walls  and  back- 
ing are  frequently  completely  immersed, 
and,  so  far  as  gravity  is  concerned,  stone 
blocks  and  rubble  become  then  trans- 
formed into  coal.  The  author,  there- 
fore, experimented  (Ex.  21)  with  some 
coal  having  a  peculiarly  "greasy"  sur- 
face, and  offering  the  advantage  of  ex- 
ceptional fluidity.  With  3  Ibs.  the  board 
moved  forward  about  1  inch,  but  no  more 
until  a  slight  jar  was  applied,  when  it 
fell;  with  4  Ibs.  a  moderate  amount  of 
vibration  also  generally  caused  failure  ; 
with  5  Ibs.  the  board  usually  moved  for- 


44 

ward  gradually  without  making  a  rush  so 
long  as  a  tolerably  considerable  amount 
of  vibration  was  maintained.  When  a 
slip  occurred  the  slope  was  invariably  If 
to  1.  The  coal  proved  to  be  more  sensi- 
tive to  vibration  than  the  wet  ballast,  and 
still  more  so  than  the  unwashed  ballast. 
Aweight  of  4  Ibs.  with  the  coal  appeared 
to  be  equivalent  to  7  Ibs.  with  the  washed 
and  3i  Ibs.  with  the  unwashed  ballast- 
The  weight  of  the  coal  being  one-half 
that  of  the  wet  ballast,  and  the  respective 
slopes  of  repose  being  If  and  1^  to  1,  the 
lateral  thrusts  would  theoretically  be  as 
16.8  :  28.7,  which  is  practically  the  exper 
iinental  result  of  4  to  7. 

X  The  author  having  occasion  -to  design 
a  solid  pier  42  feet  in  height  from  the 
bottom  of  the  harbor  to  the  surface  of 
the  quay,  where  a  soft  bottom  of  great 
thickness  and  small  consistency  precluded 
the  use  of  concrete  block  or  other  retain- 
ing wall,  adopted  an  arrangement  in 
which  an  iron  grid  of  rolled  joist,  with 
a  backing  of  large  blocks  of  rubble,  was 
substituted  for  a  wall.  It  was  necessary, 


45 

therefore,  to  know  the  lateral  thrust  of 
large  blocks  of  stone  in  such  a  structure, 
and  mistrusting  theoretical  deductions, 
the  author  made  direct  experiments  on  a 
model  to  a  scale  of  1  inch  to  the  foot. 

In  this  instance  the  individual  st<  :ies 
were  intended  to  be  fairly  uniform  in 
size,  and  of  lateral  dimensions  not  less 
than  21(T  of  the  height  of  the  wall,  so  that 
the  conditions  differed  considerably  from 
those  assumed  in  theoretical  investiga- 
tions. A  number  of  billiard  balls  exactly 
superimposed  in  a  tightly  fitting  box 
would  exert  no  thrust  though  their  slope 
of  repose  might  be  as  flat  as  3  to  1 ;  and 
it  is  not  quite  clear  how  nearly  or  re- 
motely large  boulders  in  an  iron  cage 
approximate  to  that  condition.  The 
stones  used  in  the  experiment  were 
waterworn  pieces  of  schistose  rock  hav- 
ing a  "greasy"  surface  and  a  slope  of 
repose  of  1£  to  1.  This  inclination  was 
found  by  the  author  to  obtain  in  natural 
slopes  of  all  heights,  from  the  pile  of 
metalling  by  the  roadside  to  the  hills 
themselves.  He  ascended  one  slope  of 


46 


14  to  1,  over  500  feet  in  height,  and 
found  the  balance  was  so  nearly  main- 
tained that  a  footstep  at  times  would  set 
many  tons  of  stones  in  motion;  and  a 
few  winters  ago  a  couple  of  stones  of  the 
respective  weights  of  18  tons  and  22 
tons,  descended  this  slope  and  acquired 
sufficient  momentum  to  carry  them  across 
a  road  at  the  foot  of  the  slope  and  on  to 
the  middle  of  the  lawn  in  front  of  an  ad- 
joining shooting  lodge. 

The  conditions  of  the  material  were 
thus  favorable,  as  in  the  instance  of  the 
coal,  for  obtaining  uniform  results  and  a 
maximum  lateral  thrust.  In  order  to 
exclude  all  possible  influence  from  side 
friction,  the  length  of  the  box  was  made 
four  times  the  height  of  the  wall.  AJS 
the  result  of  ten  experiments  (Ex.  22)  it 
was  found  that  a  weight  in  the  scale  cor- 
responding to  the  pressure  of  a  fluid 
weighing  10.2  Ibs.  per  cubic  foot  sufficed 
to  retain  the  rubble,  though  the  face 
planking  moved  forward  slightly  as  the 
last  few  shovelfuls  were  thrown  against 
it.  The  weight  of  the  stone  filling  was 


47 

98  Ibs.  per  cubic  foot  or  practically  the 
same  as  that  of  the  wet  ballast  last 
referred  to;  so  the  10.2  Ibs.,  or  say 
under  slight'  vibration  11  Ibs.,  in  the 
present  instance  compares  with  the  14 
Ibs.  of  the  previous  instance,  and  the  dif- 
ference is  a  measure  of  the  influence  of 
large-sized,  smooth-faced  boulders  as 
compared  with  ordinary  ballast. 

With  coal  of  the  same  size  (Ex.  23) 
the  equivalent  weight  of  fluid  was  6  Ibs. 
per  cubic  foot,  which  confirms  the  pre- 
ceding result  when  regard  is  had  to  the 
respective  weights  and  slopes  of  repose 
of  the  two  materials.  The  experiments 
collectively  proved  that  a  wall,  which 
according  to  the  ordinary  theory  would 
be  on  the  point  of  being  overturned 
by  the  thrust  of  a  bank  of  big  bould- 
ers, would  in  fact  have  a  factor  of  safety 
of  nearly  2£  to  1. 

Having  thus  briefly  reviewed  some 
direct  experiments  on  the  actual  lateral 
thrust  of  earthwork,  the  author  proposes 
to  revert  to  the  consideration  of  indirect 
experiments,  dealing  first  with  a  few  of 


48 

those  arising  on  the  34  miles  of  deep- 
timbered  trenches  and  other  works  of 
the  u underground"  railway. 

In  tunneling  very  valuable  evidence 
was  afforded  of  the  direction  of  the  line 
of  least  resistance  in  a  mass  of  earth- 
work. From  the  coming  down  of  the 
crown  bars,  the  changing  of  props,  the 
crushing  of  timber,  the  compression  of 
green  brickwork  and  other  causes,  a 
settlement  of  from  6  to  8  inches  usually 
occurred  overhead,  with  a  general  draw 
of  the  ground  towards  the  working  end 
of  the  tunnel  and  the  formation  of  fis- 
sures,, attaining  a  maximum  size  where  the 
line  of  least  resistance  cut  the  surface. 
Even  when  the  settlement  was  slight, 
fissures  were  invariably  observed  in  ad- 
vance of  the  working  end,  and  in  con- 
tinuous lines  running  parallel  with  the 
tunnel.  The  slope  of  these  fissures  was 
so  uniformly  at  the  angle  of  J  to  1,  meas- 
uring from  the  bottom  of  the  excavation 
(Ex.  24,  Fig.  5),  that  the  resident  engi- 
neer professed  to  be  able  to  foretell  with 
certainty  where  a  building  or  fence  wall, 


49 


standing  over  the  tunnel,  would  crack 
most.  Assuming  this  ^  to  1  to  represent 
Coulomb's  line  of  least  resistance,  then 
the  corresponding  natural  slope  of  repose 

Fig.5 


of  the  material  would  appear  to  be  1£  to 
1,  which  is  considerably  steeper  than 
what  it  was  in  fact. 

There  is  nevertheless  a  closer  accord 
than  usual  between  theory  and  fact  in 


50 

the  instance  of  the  several  miles  of 
fissures,  which  occurred  during  the  con- 
struction of  the  tunnels  of  the  Metro- 
politan railway.  In  other  instances,  such 
as  the  failure  of  ill- devised  timbering,  or 
the  pushing  forward  of  a  retaining  wall, 
by  heavy  clay  pressing  against  its  lower 
half,  this  accord  was  not  always  exhib- 
ited. In  some  cases  no  previous  fissures 
have  occurred,  but  a  wedge  of  1  to  1  has 
at  once  broken  off  and  gone  down  with 
the  timbering,  whilst  in  others  the 
fissure  has  appeared  immediately  at  the 
back  of  the  wall;  indeed  in  one  instance, 
for  several  consecutive  weeks,  the  author 
was  able  to  pass  a  rod,  15  feet  long,  be- 
tween the  wall  and  the  apparently  unsup- 
ported vertical  face  of  the  ground  behind 
it.  The  corresponding  theoretical  slope 
of  repose  would  thus  appear  to  be  hori- 
zontal in  one  case  and  vertical  in  the 
other,  which  is  sufficient  evidence  of  the 
necessity  of  giving  but  a  qualified  assent 
.  to  any  theoretical  deduction  affecting  the 
line  of  least  resistance  in  earthwork. 
A  very  fair  notion  of  the  relative  in- 


tensity  of  lateral  and  vertical  pressure  in 
earthwork  is  often  obtained  in  carrying 
out  headings.  The  heading  for  the  Camp- 
den  Hill  tunnel  of  the  Metropolitan  rail- 
way is  a  case  in  point  (Ex.  25).  The 
ground  consisted  of  sand  and  ballast, 
heavily  charged  with  water,  overlying 
the  clay  through  which  the  heading  was 
driven,  at  a  depth  of  44  feet  from  the 
surface.  After  the  heading  had  been 
completed  some  months,  the  clay  became 
softened  to  the  consistency  of  putty  by 
the  water  which  filtered  through  the 
numerous  fissures,  and  the  full  weight 
of  the  ground  took  effect  upon  the  set- 
tings. Both  caps  and  side  trees  showed 
signs  of  severe  stress  throughout  the  en- 
tire length  of  the  heading,  and  the  occa- 
sional fractures  in  the  roof  and  sides 
indicated  that  the  timbers  were  propor- 
tionately of  about  the  same  strength,  cr 
rather  weakness.  .  The  caps  were  of 
14-inch  square  balks,  with  a  clear  span 
of  8  feet,  and  the  sides  of  10-inch  square 
timber,  with  a  clear  span  of  9  feet.  Their 
respective  powers  of  resistance  per  square 


52 

foot  of  poling  boards,  supported,  would 

14s     103 

therefore  be  as  —t  :  —  =3J  :  1. 

8      y 

Now  if  one  thing  is  settled  by  experi- 
ence beyond  all  question,  it  is  that  the 
superficial  beds  of  London  Clay,  sodden, 
as  in  the  present  case,  with  water,  will 
not  take  a  less  slope  of  repose  than  3  to 
1.  The  average  weight  of  the  wet  ground 
over  the  heading  being  about  1  cwt.  per 
cubic  foot,  the  theoretical  lateral  press- 
ure on  the  side  trees,  at  a  mean  depth  of 
48  feet  from  the  surface,  would  be  (see 
table)  =  48  X  0.52x1  cwt.  —  25  cwt.  per 
square  foot,  and  upon  the  caps =44x1 
cwt.  =44  cwt.  per  square  foot,  or  1.76 
time  greater.  But  the  side  trees,  as  has 

been  seen,  had  on]y  -—  of  the  strength 
o.5 

of  the  caps,  so  the  irresistible  conclusion 
is  that  the  actual  lateral  pressure  of  the 
earthwork  in  this  instance  did  not  exceed 
one-half  of  that  indicated  by  theory.* 
It  is  readily  shown  that  the  full  weight 

*  See  also  "  Zur  Theorie  des  Erddrucks,"  Weyrauch 
Zeitschrift  fiir  Baukunde,  vol.  i.,  p.  192, 


53 

of  the  ground  came  upon  the  settings. 
Thus,  assuming  it  to  do  so,  the  weight 
upon  the  caps  would  be =44  cwt.  X  8  feet 
clear  span  X  3. 5  feet  distance  apart  of  the 
settings  =  1,232  cwt.,  and  taking  the 
effective  span  at  9  feet,  the  breaking 
weight,  upon  the  basis  of  Mr.  Lyster's 
experiments  on  balks  of  similar  size  and 

2  X  148/'X  2.03  cwt. 

quality,  would  be  -=-? = 

9 

1,240  cwt;  hence  the  occasional  fractures 
of  the  balks  are  fully  accounted  for.  In- 
deed, the  heading  would  have  entirely 
collapsed,  in  the  course  of  time,  had  not 
the  roof  been  supported  by  intermedi- 
ate props  practically  quadrupling  its 
strength. 

In  the  early  days  of  the  construction 
of  the  Metropolitan  railway,  a  definite 
type  of  timbering  had  not  been  arrived 
at,  and  some  remarkably  light  systems  • 
were  tried  at  times.  The  lightest  the 
author  remembers  was  the  timbering  of 
the  14-feet  wide  gullet  at  Baker  Street 
station  (Ex.  26).  Here  the  soil  cut 
through  was  made  up  of  about  8  feet  of 


54 

yellow  clay  and  gravel,  7  feet  of  loamy 
sand,  7  feet  of  sharp  sand  and  gravel, 
full  of  water,  and  4  feet  of  London  clay 
at  the  bottom  of  the  gullet.  The  timber- 
ing of  the  lower  half  consisted  of  9-inch 
by  3-inch  walings,  3  feet  apart  from 
center  to  center,  in  12-feet  lengths,  with 
f-inch  poling  boards  at  the  back.  With 
one-half  the  distributed  breaking  load, 
the  deflection  of  this  3-inch  deep  beam 
at  the  span  of  12  feet  would  be  at  least 
4  inches,  whilst  the  ultimate  deflection 
would  be  measured  by  feet.  As  the 
walings  did  not  bend  nearly  as  much  as 
4  inches,  it  will  be  a  liberal  estimate  to 
assume  that  the  actual  lateral  pressure  of 
the  earthwork  was  equal  to  half  the  dis- 
tributed breaking  weight  of  the  wal- 
ing. Having  reference  to  the  quality  of 
the  timber,  this  may  be  estimated  at 

32"x9"x2.6cwt. 

__ _ _ =  17.6  cwt. ;  and  since 

12   0 

the  area  of  the  poling  boards  supported 
by  each  waling  was  36  square  feet,  it 
follows  that  the  lateral  pressure  of  the 
earthwork  could  not  have  exceeded  55 


55 

Ibs.  per  square  foot.  But  the  depth  of 
the  bottom  waling  below  the  surface 
was  23  feet,  or,  neglecting  the  clay,  and 
taking  only  the  sharp  sand  and  ballast 
charged  with  water,  the  depth  would 
still  be  20  feet,  and  the  weight  of  fluid 
corresponding  to  the  55  Ibs.  per  square 
foot  pressure  no  more  than  2.75  Ibs.  per 
cubic  foot. 

It  will  be  remembered  that  the  natural 
slope  of  the  sand  and  ballast  in  Lieuten  - 
ant  Hope's  retaining- wall  experiments 
was  about  1J  to  1,  and  that  the  actual 
and  theoretical  corresponding  fluid 
pressures  were  respectively  10.3  Ibs.  and 
23.6  Ibs.  per  cubic  foot.  In  the  case  of 
the  gullet,  the  natural  slope  of  the  ballast 
and  sand  would  similarly  be  not  less  than 
1J  to  1,  and  yet  the  fluid  pressure  could 
not  have  exceeded  2.75  Ibs.  This  one 
fact,  therefore,  is  sufficient  to  prove  that 
the  universal  assumption  of  the  pressure 
of  earthwork  being  analogous  to  thut  of 
fluid,  and  proportional  to  the  depth,  is 
one  of  convenience  rather  than  truth. 
The  explanation  of  the  singularly  small 


56 

lateral  thrust  of  the  ballast  in  the  present 
case  is  to  be  found  in  the  fact  that  the 
ballast  was  lying  between,  and  partially 
held  back  by,  the  two  relatively  tenacious 
layers  of  loamy  sand  and  clay.  As  an 
extreme  example  of  the  same  kind  of 
action  (Ex.  27),  the  author  may  state 
that  he  once  applied  to  a  wooden  box 
full  of  sand  a  pressure  equivalent  to  a 
column  of  that  material  1,400  feet  high 
before  the  box  burst.  On  the  fluid 
hypothesis,  the  lateral  pressure  would 
have  been  1,400  feet  X  23.6  Ibs.,  or 
about  15  tons  per  square  foot;  but  of 
course  a  few  Ibs.  would  have  burst  the 
box,  and  the  sand  was  retained  by  being 
jammed  between  the  bottom  and  lid 
of  the  little  deal  box — the  equivalents  of 
the  tenacious  strata  in  the  gullet. 

In  shafts  the  stress  on  the  timbering- 
is  far  less  than  in  a  continuous  trench  or 
heading,  by  reason  of  the  frictional  ad- 
hesion and  tenacity  of  the  adjoining 
earth.  Thus  (Ex.  28)  at  a  depth  of  be- 
tween 40  and  50  feet,  10-inch  square 
timbers,  4  feet  6  inches  apart,  proved  of 


57 

ample  strength  to  support  the  sides  of  a 
12-feet  square  shaft,  though  the  same 
sized  timbers  at  the  reduced  distance  of 
3  feet  6  in-ches  apart,  failed,  as  has  been 
seen,  to  support  the  sides  of  a  9-feet 
square  heading. 

After  the  experience  of  several  rather 
troublesome  slips,  light  timbering  was 
abandoned,  and  a  type  which  proved  to 
be  of  ample  strength  to  meet  all  the  con- 
tingencies of  heavy  ground,  vibration 
from  road  traffic,  and  the  surcharge  of 
lofty  buildings,  was  adopted.  In  this 
type  (Ex.  29)  the  14-feet  walings  in- 
creased in  scantling  to  12  inches  by  7 
inches,  were  spaced  7  feet  apart,  and 
strutted  at  each  end  and  at  the  center. 
At  one-half  the  breaking  weight  the  sup- 
porting power  of  the  walings  would  be 

,    72Xl2x3cwt.  X  112  „ 

about-  65ax7         —=670  Iba. 

per  square  foot,  and  as  the  depth  of  the 
excavation  was  in  some  instances  as 
much  as  36  feet,  this  would  correspond 
to  a  fluid  pressure  of  18.6  Ibs  per  cubic 
foot.  With  ground  weighing  112  Ibs. 


58 

per  cubic  foot,  and  a  slope  of  repose  of 
1-J  to  1,  the  theoretical  lateral  pressure 
would  be  32  Ibs.  per  cubic  foot;  and 
when  it  is  remembered  that  this  does 
not  include  any  allowance  for  the  sur- 
charge due  to  contiguous  buildings,  and 
that  the  stress  on  the  timber  is  taken  at 
fully  half  the  breaking  weight,  it  is  clear 
that  the  average  actual  lateral  pressure 
of  the  earthwork  must  have  been  less 
than  half  that  indicated  by  theory. 

On  the  extensions  of  the  Metropolitan 
railway,  the  same  type  of  timbering  was 
adopted,  but  the  walings  were  generally 
9  inches  by  4  inches,  and  spaced  3  feet 
apart.  The  supporting  power,  upon  the 
same  basis  as  in  the  last  instance,  would 
be  about  430  Ibs.  per  square  foot.  In 
most  cases  this  strength  proved  to  be 
sufficient,  but  in  a  few  instances  the 
walings  broke,  or  showed  such  signs  of 
distress  that  additional  support  had  to 
be  given.  This  was  the  case  in  some  of  the 
deep  trenches  along  the  Thames  Em- 
bankment, where  heavy  wet  silt  was 
traversed.  Near  Whitehall  Stairs  (Ex. 


59 

30)  the  trenches  were  40  feet  deep,  so 
the  elastic  strength  of  the  timbering  was 
only  adequate  to  the  support  of  a  fluid 
pressure  of  10.6  Ibs.  per  cubic  foot,  or 
probably  but  one  fourth  of  that  theo- 
retically due  to  the  material ;  it  is  there- 
fore no  matter  for  surprise  that  the  wal- 
ings  proved  unequal  to  their  work.  The 
stability  of  the  timbering  in  more  moder- 
ate depths  was,  on  the  other  hand,  con- 
firmatory of  the  general  deductions 
drawn  from  previous  examples  as  to  the 
wide  divergence  between  the  actual  and 
calculated  thrust  of  earthwork. 

Turning  now  from  the  consideration 
of  the  temporary  works  of  timbering  to 
the  finished  and  permanent  structures  on 
the  underground  railway,  a  similar  varia- 
tion in  strength  will  be  found  to  obtain. 
The  lightest  retaining  wall  on  the  line  is 
that  at  the  Edgware  Koad  station  yard 
(Ex.  31,  Figs.  6  and  7).  This  wall  is  23 
feet  in  height  from  the  top  of  the  footing 
to  the  ground  level,  and  has  a  maximum 
thickness  of  but  6  feet  3  inches  at  the 
base,  out  of  which  has  to  be  deducted  a 


60 

panel  2  feet  6  inches  deep.  Calculating 
the  moment  of  stability  at  the  level  of 
the  footings  and  round  a  point  3  inches 
back  from  the  face  of  the  pier — which  is 
a  sufficient  allowance  for  the  crushing 
action  on  the  brickwork — M=4.4  feet,  X 
8,800  Ibs.  =  38,720  foot  pounds  per 


Fig.6 


lineal  foot  of  wall.     Dividing  by  — ,  then 

19^1bs.  per  cubic  foot  is  the  weight  of  the 
fluid^which  would  overturn  this  retaining 
wall.  The  ground  supported  is  light 
dry  sand,  having  a  slope  of  repose  of 
about  1J  to  1,  and  consequently  exert- 
ing a  theoretical  lateral  thrust  equivalent 


61 

to  a  24-]b.  fluid.  There  is  practically  no 
tenacity  in  the  soil,  as  the  author  re- 
members seeing  demonstrated  on  one 
occasion  when  a  horse  and  cart,  ap- 
proaching too  near  the  top  edge  of  the 
slope,  broke  it  away  and  rolled  together 
to  the  bottom  of  the  23 -feet  cutting. 
Although  theoretically  deficient  in  stabil- 
ity, and  subject  to  heavy  vibration  from 
the  two  minutes  train  service,  the  wall 
has  stood  perfectly  without  exhibiting 
the  slightest  movement.  Upon  the  basis 
of  the  results  of  actual  experiments,  and 
having  reference  to  the  character  of  the 
soil  and  other  conditions,  the  factor  of 
safety  would  appear  to  be  about  2  to  1. 

A  far  lower  factor  sufficed  to  secure 
the  temporary  stability  of  the  dry  areas 
at  the  station  buildings  previous  to  the 
erection  of  the  arched  roofs  (Ex.  32). 
The  arrangement  at  Sloane  Square  sta- 
tion is  shown  in  Figs.  8  and  9.  The 
joint  stability  of  the  front  and  back 
walls  is  the  same  as  that  of  a  solid 
rectangular  wall  having  a  thickness  equal 
to  (2.4  X  2.5  4-  3.8  X  5.2)*  =  5.1  feet,  or 


62 


say  £  of  the  height.  A  fluid  pressure  of 
about  9  Ibs.  per  cubic  foot  would  upset 
such  a  wall,  so  the  factor  of  safety,  until 


the  arched  roof  abutted  against  the  dry 
areas,  was  only  that  due  to  the  few 
14 -inch  brick  arches  which  tied  the  walls 
together  with  a  certain  amount  of  rigid 


ity.  This  result  would  perhaps  have 
surprised  the  author  more  had  he  not 
previously  investigated  many  cases  of 
old  timber  wharves,  in  which  the  piles 
and  plauking  had  lost  more  than  f  of 
their  original  strength  from  decay,  and 


Section  on  line  A  B. 

yet  held  on  against  a  theoretically  over- 
powering thrust  of  earthwork. 

The  relatively  strongest  wall  on  the 
Metropolitan  railway  system  is  at  the 
St.  John  s  Wood  Koad  station  (Ex.  33), 
and  that  has  given  considerable  trouble. 
Though  8  feet  6  inches  thick  at  the  base. 


64 


and  backed  up  to  a  height  of  16  feet 
only  out  of  the  total  height  of  21  feet  6 
inches,  and  supported  at  the  top  by  the 
thrust  of  the  arched  roof  of  the  station, 
this  wall  moved  over  and  forward  to  an 
extent  which  necessitated  the  immediate 
adoption  of  remedial  measures. 

The   moment   of    stability   per   lineal 
foot     M=  73,000     foot  pounds,     conse- 

1  na 

quently  dividing  by  -^-  the  fluid  resist- 

ance is  107  Ibs.  per  cubic  foot,  or  allow- 
ing for  the  thrust  of  the  arched  roof  of 
the  station,  considerably  greater  than 
that  of  a  perfect  fluid  having  the  same 
density  as  the  ground  supported.  It  is 
not  contended  that  such  a  pressure  ever 
occurred  upon  the  wall,  although  the 
ground  is  heavy  yellow  clay.  The  fail- 
ure arose  from  causes  which  will  be  re- 
ferred to  more  generally  hereafter,  and 
the  case  is  only  mentioned  as  a  signal 
instance  of  the  futility  of  hoping  to  re- 
duce the  engineering  of  retaining  walls 
to  the  form  of  a  mathematical  equa- 
tion. 


65 

It  is  a  suggestive  fact  that,  out  of  the 
9  miles  of  retaining  wall  on  the  under- 
ground railway,  the  exceptionally  weak 
wall  should  show  no  movement  either 
during  or  after  construction,  whilst  the 
exceptionally  strong  wall,  though  having 
six  times  the  stability  of  the  former, 
should  fail.  If  an  engineer  has  not  had  f 
some  failures  with  retaining  walls,  it  is  | 
merely  evidence  that  his  practice  has  not  | 
been  sufficiently  extensive ;  for  the  at- 
tempt to  guard  against  every  contin- 
gency in  all  instances  would  lead  to  ruin- 
ous and  unjustifiable  extravagance,  and 
be  indeed  as  ridiculous  a  preceding  as 
the  making  every  soft  clay  cutting  at  a 
slope  of  10  to  1*  because  in  a  few  places 
such  cuttings  happen  to  slip  down  to 
that  slope. 

In  two  instances  comparatively  heavy 
retaining  walls  have  failed  on  the  Metro- 
politan railway.  During  the  construc- 
tion of  the  line,  the  wall  on  the  west 
side  of  the  Farringdon  street  station, 
(Ex.  34),  failed  bodily  by  slipping  out  at 
the  toe  and  falling  backwards  on  to  the 


67 


slope  of  the  earthwork  (Fig.  10).  This 
wall  (Figs.  11  and  12)  was  29  feet  3  in- 
ches high  above  the  footings,  and  8  feet 


bb 


to 


»«w»»?i..N^i  -M---   ,o  :•. '.^ :•.•••-•• -I   . 


--£"62 •*-; 


6  inches  thick.  The  ground  consisted 
of  about  17  feet  of  made  ground,  3  feet 
of  loamy  gravel,  and  9  feet  of  clay.  At 
a  distance  of  15  feet  from  the  back  of 


68 

the  wall,  and  at  a  depth  of  15  feet  from 
the  surface  of  the  road,  was  the  Fleet 
Sewer — a  badly  constructed  and  much 
broken  brick  barrel,  10  feet  6  inches  di- 
ameter and  3  rings  thick.  It  was  be- 
lieved that  the  leakage  from  the  sewer 
induced  the  failure  of  the  wall,  but  in 
reconstruction  both  wall  and  sewer  were 
strengthened.  The  latter  was  made  4 
rings  thick  in  cement,  and  the  former 
(Ex.  35)  was  increased  in  thickness  to  12 
feet  9  inches  (Figs.  13  and  14).  Origi- 
nally the  stability  wras  equal  to  the  resist- 
ance of  a  fluid  pressure  of  24  Ibs.  per 
cubic  foot,  and,  as  reconstructed,  to  54 
Ibs. 

On  the  opposite  side  of  the  same  sta- 
tion yard  the  ground  was  retained  by  a 
line  of  vaults  (Ex.  36,  Figs.  15  and  16), 
29  feet  high  above  the  footings,  and  17 
feet  deep — or  double  the  original  thick- 
ness of  the  wall  last  referred  to.  Al- 
though the  resistance  to  overturning  was 
greater  in  the  proportion  of  62  Ibs.  to  24 
Ibs.  per  cubic  foot,  the  vaults  some  years 
after  construction  came  over  15  inches 


69 


at  the  top,  and  slid  forward  considerably 
more.     The  movement  when  once  fairly 


commenced  was  rapid  and  alarming,  as  a 
mass  of  densely   inhabited   houses  was 


70 

within  20  feet  of  the  back  of  the  vaults. 
Steps  were  promptly  taken  to  strengthen 
the  work,  by  building  intermediate  piers 
and  doubling  the  thickness  at  the  back 
(Ex.  37,  Fig.  17).  This  arrested  the 
movement  for  a  few  months,  when  the 
vaults,  whose  stability  had  been  thus  in- 
creased to  93  Ibs.  per  cubic  foot,  again 
began  to  go  over  and  slide  forward.  It 
was  clear  that  mere  weight  would  not 
insure  stability,  so  3 -feet  square  brick 
struts  were  carried  at  intervals  from  the 
toe  of  the  piers  across  and  under  the 
railway  to  the  retaining  wall  of  the  low- 
level  line  traversing  the  station  yard,  at 
a  distance  of  about  34  feet  from,  and  8 
feet  below,  the  level  of  the  footings  of 
the  vaults. 

The  soil  in  the  preceding  instance  con- 
sisted of  about  12  feet  of  made  ground 
overlying  the  clay,  and,  as  in  the  former 
case,  a  sewer  was  to  be  found  rather 
close  to  the  back  of  the  work.  West- 
ward of  the  vaults,  the  clay  encountered 
in  the  construction  of  the  line  was  hard 
blue  clay,  requiring  the  use  of  a  pick, 


71 

and  portions  of  the  temporary  cuttings 
in  the  station  yard,  on  the  site  where 
the  vaults  were  subsequently  built,  stood 
fairly  for  many  months  at  a  slope  of  1^-  to 
1.  At  one  point,  however,  troublesome 
slips  occurred,  and  even  a  2  to  1  slope 
had  to  be  piled  at  the  toe  to  prevent  for- 
ward movement.  It  was  at  this  point 
that  the  vaults  were  subsequently  found 
to  be  most  dislocated. 

In  neither  of  the  above  cases  was  fail- 
ure due  to  a  deficient  moment  of  stabili- 
ty in  the  wall,  and  therefore  the  fact  of 
their  failure  does  not  in  any  way  conflict 
with  the  results  of  the  experiments  pre- 
viously set  forth.  In  each  case  water  at 
the  back  of  the  wall  was  as  usual  the 
active  agent  of  mischief — not  in  thrust- 
ing the  wall  forward  by  hydrostatic  press- 
ure, but  in  softening  the  clay  and  afford- 
ing a  lubricant,  so  that  the  resistance 
was  reduced  to  a  sufficient  extent  to  en- 
able the  otherwise  innocuous  lateral 
pressure  of  the  earthwork  to  tilt  and 
thrust  forward  the  wall. 

A  costly,  but  conclusive,  experience  of 


72 

this  softening  action  was  obtained  in  the 
instance  of  the  central  pier  to  the  double 
covered  way  on  the  District  railway  near 
Gloucester  Eoad  station  (Ex.  38).  The 
weight  per  lineal  foot  was  21  tons  per 
foot  run  of  pier,  and  4  feet  9  inches  was 
spread  out  by  footings  and  concrete  to  a 
base  of  10  feet;  hence  the  pressure  on 
the  ground  was  2.1  tons  per  square  foot. 
In  a  similar  construction  near  Alders  - 
gate  the  load  was  25  tons,  and  the  width 
of  base  8  feet  3  inches,  giving  the  in- 
creased load  of  2.9  tons  upon  the  founda- 
tions. At  the  Smithfield  market  the 
author  did  not  hesitate  to  place  a  column 
carrying  435  tons  upon  a  12-feet-square 
base,  which  is  equivalent  to  a  load  of  3 
tons  per  square  foot ;  and  in  the  Euston 
Eoad,  the  side  wall  of  the  covered  way 
has  a  load  of  15  tons  per  lineal  foot  on  a 
4 -feet-wide  base,  which  is  at  the  rate  of 
3 1  tons  per  square  foot.  In  all  in- 
stances the  foundation  was  clay,  of  ap- 
parently equal  solidity,  and  in  every  in- 
stance but  the  first  no  settlement  at  all 
occurred.  For  some  years  no  settlement 


73 

was  observable  in  that  case  either,  but 
ultimately,  after  an  accidental  flooding 
of  the  line,  and  permanent  accumulation 
of  water  near  the  foundations,  owing  to 
the  line  being  below  the  limits  of  nat- 
ural drainage,  and  the  pumping  being 
neglected,  cracks  were  observed  in  the 
arches,  and  on  examination  the  concrete 

Frg.18 


and  footings  of  the  central  pier  were 
found  to  be  fractured,  as  shown  in  Fig. 
18.  The  load  of  21  tons  per  lineal  foot 
was  thus  imposed  upon  a  base  only  4 
feet  wide,  and  the  softened  clay  proved 
unable  to  sustain  the  pressure  of  up- 
wards of  5  tons  per  square  foot.  Con- 
siderable difficulty  was  experienced  in 


74 

checking  the  movement  when  once  es- 
tablished. The  center  pier  was  under- 
pinned with  brickwork  in  cement,  but  the 
footings,  though  of  exceptional  strength, 
were  again  sheared  off,  and  it  was  found 
necessary  to  use  6-inch  York  landings. 

This  failure  shows  the  advisability  of 
making  concrete  foundations  of  sufficient 
transverse  strength  to  distribute  the 
weight  uniformly  over  the  ground.  As 
the  result  of  experiment,  the  author  is 
of  opinion  that  the  ultimate  tensile  re- 
sistance in  a  beam  of  good  cement  or 
lias  lime  concrete,  is  about  100  Ibs.  per 
square  inch,  and  in  a  beam  of  good 
brickwork  in  cement  as  much  sometimes 
as  350  Ibs.  per  square  inch.*  Taking 
the  former  value  (Ex.  39),  a  12-inch  thick 
concrete  foundation,  projecting  12  inches 
from  the  face  of  a  wall,  would  break 

with  a  distributed  loader =4,800 

6x6 

Ibs.;  or,  say,  2  tons  per  square  foot. 
With  a  pressure  upon  the  foundation  of, 

*  Vide  "The  Strength  of  Brickwork."  By  B.  Baker. 
'Engineering,"  vol.  xiv. 


75 


say,  3  tons  per  square  foot,  and  a  factor 
of  safety  of  2,  the  thickness  of  a  con- 
crete foundation  would  therefore  be 


3  tons  X  2 

^=1.73  time  the  amount  of 


2  tons 

its  projection  beyond  the  face  of  the 
pier  or  wall,  and  the  author  would  not 
advise  a  less  thickness  being  used  when 
the  foundation  rests  on  plastic  clay. 

Water  naturally  gravitates  to  the 
foundation  of  a  retaining  wall,  and  a 
softening  occurs.  Owing  to  the  lateral 
thrust  of  the  earthwork,  the  pressure  on 
the  foundations  is  not  uniform,  and  in- 
stead of  settling  uniformly,  the  outer 
edge  descends  fastest  and  the  top  of  the 
wall  is  thrown  outwards.  The  same 
softening  reduces  the  clay  to  a  condition 
in  which  it  is  easily  ploughed  up  by  the 
advancing  wall,  and  the  water  acts  as  an 
admirable  lubricant  in  diminishing  the 
friction  between  the  bottom  of  tlie  wall 
and  the  clay  on  which  it  rests.  These 
elements  are  exceedingly  variable  in  their 
nature,  and  it  is  practically  i 


76 

foretell  the  extent  of  their  influence  in 
each  individual  case. 

In  tunneling,  clay  may  be  the  best  or 
the  worst  of  materials — almost  self-sup- 
porting, or  pressing  with  irresistible 
force  on  the  crushing  timbers  and  brick- 
work. It  may  be  taken  for  granted  that 
in  good  ground  bad  work  will  occasion- 
ally creep  into  tunneling,  however  close 
the  inspection.  How  good  a  material 
clay  can  be  was  enforced  upon  the 
author's  attention  once  in  renewing  a 
short  length  of  defective  tunnel  lining, 
when  on  cutting  down  the  work  it  was 
found  that  for  some  50  feet  the  side  wall, 
instead  of  being  2  feet  6  inches  thick  as 
intended,  consisted  merely  of  a  skin  of 
brickwork  9  in  inches  thick  on  the  face, 
with  a  number  of  dry  bats  thrown  in 
loosely  behind  this  thin  face  wall  to  fill 
up  the  space  excavated.  This  tunnel 
(Ex.  40)  was  loaded  with  a  weight  of  46 
feet  of  clay  over  the  crown,  but  no  meas- 
urable settlement  had  taken  place  ten 
years  after  completion,  and  it  was  rather 
by  sounding  the  side  wall,  than  by  the 


77 

observance  of  cracks,  that  a  suspicion 
was  raised  as  to  its  solidity.  If  the  full 
weight  of  the  ground  had  come  upon  the 
tunnel  as  it  did  upon  the  heading  (Ex. 
25),  the  pressure  upon  the  side  wall 
would  have  been  45  tons  per  lineal  foot, 
or  practically  double  the  strength  of  the 
9-inch  work  as  determined  by  experi- 
ment. 

Of  course  the  clay  in  this  case  was 
hard  blue  clay,  which  had  not  been  af- 
fected by  the  action  of  air  and  moisture. 
As  explained  by  the  Rev.  J.  C.  Clutter- 
buck,  many  years  ago,  the  superficial  lay- 
ers of  the  London  clay  are  yellow,  because 
the  protoxide  of  iron  is  changed  into  a 
peroxide  by  the  action  of  air  and  moist- 
ure in  the  disintegrated  mass,  and  it  is 
the  yellow  clay,  therefore,  which  is  the 
dread  of  the  engineer.  As  good  an  ex- 
ample as  any  of  the  difference  between 
the  two  materials  was  afforded  forty* 
years  ago,  in  the  well  known  slip  which 
occurred  in  the  75-feet-deep  cutting  at 
New  Cross,  when  nearly  100,000  tons  of 
yellow  clay  slipped  forward  on  the  hard 


78 


smooth  surface  of  the  shale — like  under- 
lying blue  clay,  and  buried  the  entire 
line  for  a  length  of  more  than  a  hundred 
yards  to  a  depth  of  12  feet.* 

Owing  to  a  misunderstanding,  a  sec- 
tion of  concrete  wall  designed  by  the 
author  to  form  one  side  of  a  running 
shed,  and  to  retain  the  earthwork  in  a 
13-feet  cutting  through  light-made 
ground,  was  adopted  also  in  a  similar 
case,  but  where  the  ground  was  heavy 
wet  clay,  and  the  cutting  30  feet  deep 
(Ex.  41).  A  wall  13  feet  in  height  from 
formation  to  coping,  and  only  3  feet  3 
inches  thick  at  the  base,  had  thus  to  sus- 
tain a  surcharge  of  17  feet.  As  the 
slope  of  repose  was  at  least  1-|  to  1,  the 
lateral  thrust  was  theoretically  equiva- 
lent to  a  fluid  pressure  of  about  70  Ibs. 
per  cubic  foot,  whereas  a  pressure  of  less 
than  one- third  that  intensity  would  have 
overturned  the  wall.  The  latter,  never- 
theless, held  up  the  ground  fairly  for 
some  months,  though  the  nature  of  the 


*  Vide  Minutes  of  Proceedings  Inst.  C.E..  vol.  iii.,  p. 
139. 


79 


soil  was  such  that  it  ultimately  became 
necessary  to  add  strong  counterforts  to 
the  wall,  and  to  reduce  the  slope  of  the 
cuttings  generally  to  2  :  1. 

On  the  Thames  Embankment  heavy 
clay  filling  was  in  places  cut  through  by 
the  District  railway,  and  in  several  in- 
stances the  light  side  walls  of  the  cover- 
ed way  were  thrust  over  a  few  inches  at 
the  top  before  the  girders  were  bedded 
(Ex.  42).  The  side  walls  were  eighteen 
feet  in  height  from  the  invert  to  the 
ground  level,  and  5  feet  6  inches  thick, 
with  panels  5  feet  6  inches  wide,by  about  2 
feet  9  inches  deep,and  piers  2  feet  6  inches 
wide.  A  fluid  pressure  of  16  Ibs.  would 
overcome  the  stability  of  these  walls,  but, 
though  subject  to  the  pressure  of  the 
heavy  clay  filling,  none  of  them  failed. 
The  existence  of  an  undue  pressure  was, 
however,  manifested  by  the  thrusting  for- 
ward of  the  green  brickwork  during  the* 
few  weeks  that  the  walls  were  left  unsup- 
ported by  the  girders. 

The  retaining  walls,  at  the  approach 
to  Euston  Station,  afford  a  good  illustra- 


80 

tion  of  the  impossibility  of  making  any 
reasonable  approximate  estimate  of  the 
possible  lateral  thrust  of  yellow  clay,  or 
of  stating  positively  that  no  movement 
will  ever  occur.  These  walls,  soon  after 
construction,  were  forced  out  in  an  ir- 
regular way  at  the  top,  bottom,  or 
middle,  but  on  pulling  them  down,  the 
clay  behind  appeared  to  be  free  from  fis- 
sures and  to  stand  vertical.  Cast-iron 
struts  were  subsequently  put  in  between 
the  opposite  retaining  walls ;  and  al- 
though General  Burgoyne,  who  had 
given  much  attention  to  the  subject  of 
revetments,  prophesied  at  the  time  that 
they  would  be  removed  in  a  few  years, 
"  when  the  ground  had  become  consoli- 
dated," the  struts  still  remain,  and  the 
walls  still  give  signs  of  severe  and  in- 
creasing stress. 

It  is  not  only  London  clay  that  proves 
so  embarrassing  to  engineers.  In  a  re- 
cent Paper*  particular  attention  was 
called  to  the  treacherous  nature  of  some 

*Vide  "Earthwork  Slips,"  Minutes  of  Proceedings 
Inst.  C.  E,,  vol.  Ixiii,,  p.  280  et  seq. 


81 

boulder  clay  which,  "  although  so  tough 
and  tenacious  as  to  give  the  utmost  dif- 
ficulty in  excavation,  after  a  short  ex- 
posure became  soft  and  pasty  in  the 
winter,  often  jolting  down  the  slurry." 
Examples  were  given  of  formidable  slips 
in  this  material,  in  contrast  with  which 
the  author  would  point  to  the  comparati- 
vely slow  wasting  of  the  huge  boulder  clay 
cliffs  near  the  mouth  of  the  Tyne,  a  mat- 
ter which  he  had  occasion  to  investigate 
very  closely  in  connection  with  the  Duke 
of  Northumberland's  Jands  in  that  dis- 
trict. From  a  comparison  of  surveys 
extending  over  a  period  of  one  hundred 
and  fifty  years,  it  appeared  that  the 
wasting  of  the  cliff  was  very  slow,  and 
due  solely  to  the  wash  of  the  waves  at  its 
base.  At  no  time  was  the  slope  of  repose 
of  this  105 -feet-high  cliff  more  than  1  to 
1,  and  in  places  it  stood  for  years  at  an 
average  slope  of  less  than  £  to  1.  With, 
his  experience  of  North  London  clay,  the 
author  was  startled  to  find  people  con- 
tentedly living  in  houses  partially  over- 
hanging the  brow  of  this  steep  and 


82 

ragged  cliff,  but  the  stability  of  the  clay 
was  so  great,  and  the  wasting  so  uniform, 
that  the  fact  of  the  outhouses  being  at 
the  bottom  of  the  100  feet  slope,  and  the 
main  building  at  the  top,  did  not  appear 
in  any  way  to  disturb  the  equanimity  of 
the  householders. 

The  failures  of  dock  walls,  though  nu- 
merous and  instructive,  afford  no  direct 
evidence  as  to  the  actual  lateral  pressure 
of  earthwork,  because  in  practically  every 
instance  the  failure  is  traceable  to  defec- 
tive foundations.  The  author  cannot  re- 
call any  case  in  which  a  dock  or  quay 
wall  founded  on  rock  has  overturned  or 
moved  forward,  though  on  other  founda- 
tions a  movement  to  a  greater  or  lesser 
extent  is  so  much  the  rule  that  Voisin 
Bey,  the  distinguished  engineer-in-chief 
of  the  Suez  Canal,  once  stated  to  the 
author  that  he  could  name  no  exception 
to  it,  since  he  had  failed  to  find  any  long 
line  of  quay  wall,  which  on  close  inspec- 
tion proved  to  be  perfectly  straight  in 
line  and  free  from  indications  of  move- 
ment. A  brief  examination  of  some  in- 


83 

stances  of  the  failures  of  dock  walls  will 
show  how  powerfully  unknown  practical 
elements  affect  theoretical  deductions  in 
such  cases. 


A  well-known  and  often  cited  case  is 
that  of  the  original  Southampton  dock 
wall,  constructed  now  some  forty  years 
ago  (Ex.  43,  Fig.  19).  This  wall,  38  feet 
in  height  from  the  foundation  to  the 
coping,  was  built  on  a  platform  of  6 -inch 
planks,  resting  on  a  sandy  and  loamy 
bottom.  Before  the  water  had  been  let 


84 

into  the  dock,  or  the  backing  carried  to 
the  full  height,  the  wall  moved  forward 
in  some  places  as  much  as  three  feet,  but 
came  over  hardly  anything  at  the  top. 
When  the  water  was  let  into  the  dock, 
the  filling  behind  becoming  saturated, 
the  pressure  on  a  receding  tide  exag- 
gerated, and  to  secure  stability  it  was 
found  necessary  to  discontinue  the  filling 
at  some  distance  below  the  full  height 
of  the  wall,  and  to  substitute  a  timber 
platform. 

The  thickness  of  this  wall  at  the  base 
is  32  per  cent,  of  the  height  between  the 
buttresses,  45  per  cent,  at  the  buttresses, 
and  a  rectangular  wall  containing  the 
same  quantity  of  material  would  have  a 
thickness  equal  to  26  per  cent,  of  the 
height.  Though  the  base  is  wide,  the 
weight  is  light  as  compared  with  most 
other  dock  walls,  and  the  tendency  to 
slide  forward  is  therefore  greater.  If 
founded  on  a  rock  bottom,  a  fluid  press- 
ure of  about  40  Ibs.  per  cubic  foot  would 
have  been  required  to  overturn  the  wall, 
but  of  course  a  fraction  of  this  pressure 


85 

would  suffice  to  make  it  move  forward  on 
the  actual  bottom. 

The  conclusion  drawn  by  Mr.  Giles, 
M.  Inst.  C.  E.,  the  engineer  of  the  docks, 
from  this  and  other  failures  is,  that  the 
quality  of  a  dock  wall  is  of  little  conse- 


Fig.20 


quence  compared  with  the  quantity,  and 
that  it  ought  to  be  sufficiently  strong  not 
only  to  hold  any  amount  of  any  kind  of 
backing  put  against  it,  but  to  carry  a 


86 

head  of  water  equal  to   its  height  if  it 
were  left  dry  on  the  other  side.* 

These  principles  have  been  adhered  to 
in  the  recent  extension  of  the  South- 
ampton docks  (Ex.  44,  Fig.  20).  Here 
the  wall  is  founded  on  a  mass  of  con- 
crete 21  feet  wide;  the  effective  thickness 
at  base  is  about  45  per  cent,  and  the 
mean  thickness  41  per  cent,  of  the  height. 
A  fluid  pressure  of  from  60  to  70  Ibs. 
would  be  required  to  overturn  this  wall 
if  on  a  hard  foundation,  and  probably  as 
much  to  make  it  move  forward,  unless 
the  bottom  were  of  clay  or  of  other  un- 
favorable material.  Mr.  Giles  has  found 
even  a  heavier  wall  slide,  when  founded 
on  a  thin  layer  of  gravel  overlying  clay. 
In  the  earlier  wall,  if  the  co-efficient  of 
friction  of  the  base  on  the  ground  were 
less  than  f ,  the  wall  would  slide  ratner 
than  overturn  ;  but  in  the  latter  wall, 
without  buttresses,  any  co-efficient  ex- 
ceeding ^  would  be  sufficient  to  prevent 
sliding. 

*  Vide  Minutes  of  Proceedings  Inst.  C.  E.,  vol.  lv., 
p.  52. 


87 


For  comparison  with  the  above,  the 
section  of  the  east  quay  wall  of  the 
Whitehaven  dock  may  be  next  referred 
to  (Ex.  45,  Fig.  21).  Having  the  same 
height  as  the  Southampton  dock  wall, 
the  thickness  at  the  base  is  but  37  per  / 


cent  of  the  height,  the  mean  thickness  31 
per  cent.,  and  the  concrete  foundation  16 
feet  6  inches,  instead  of  21  feet  wide. 
This  wall  has  stood  perfectly,  though  it 
would  fail  to  resist  the  head  of  water 
mentioned  by  Mr.  Giles,  but  would  be 


88 

overturned  by  a  fluid  weighing  from  45 
to  50  Ibs.  per  cubic  foot.  During  con- 
struction, weep  holes  were,  however,  left 
in  the  walls  to  relieve  them  of  hydro 
static  pressure. 

Fig.22 


.10.6-^' 


Another  dock  wall  of  the  same  height 
as  the  preceding  ones,  is  that  of  the 
Avonmouth  dock  (Ex.  46,  Fig.  22).  In 
this  instance  the  thickness  is  42  per  cent, 
of  the  height,  and  the  concrete  base  22 
feet  6  inches  wide,  dimensions  which, 


89 

with  a  good  foundation,  would  enable  the 
wall  to  stand  a  full  hydrostatic  pressure 
at  the  back.  Owing  to  the  treacherous 
nature  of  the  bottom,  a  long  length  of 
this  wall  nevertheless  slipped  forward  at 
one  point  as  much  as  12  feet  6  inches, 
and  sunk  4  feet  6  inches  without  the 
latter  being  affected,  whilst  at  another 
point,  where  there  was  no  for  ward  move- 
ment, the  wall  came  over  about  1  foot  8 
inches.  When  the  failure  occurred,  the 
foundation  rested  on  apparently  stiff  blue 
clay,  but  in  subsequent  portions  the 
concrete  was  carried  down  through  the 
clay  to  the  sand.*  On  the  east  side  of 
the  dock,  though  the  walls  were  founded 
at  an  average  depth  of  no  less  than  9 
feet  below  the  bottom  of  the  dock,  they 
still  moved  forward  in  the  mass  some  15 
feet  6  inches,  and  sunk  7  feet  6  inches, 
The  filling  was  carefully  punned  in 
layers,  with  material  which  seems  to  have 
stood  fairly  at  a  slope  of  1J  or  2  to  1,  so 
that  the  wall  theoretically  possessed  an 

*  Vide  Minutes  of  Proceedings  Inst.  C.  E.,  vol.  lv., 
p.  15. 


90 

excess  of  strength,  and  yet,  owing  to  the 
existence  of  conditions  which  it  was  im- 
possible for  the  engineer  to  foresee, 
failures  occurred  as  described. 

A  somewhat  similar  case  of  sliding  for- 
ward occurred  at  the  New   South  Dock, 

Fifl.23 


: 


a^m 

^ JLO.- * 


West  India  Docks  (Ex.  47,  Fig.  23).  The 
wall  is  35  feet  9  inches  high  from  the  top 
of  the  footings  to  the  coping,  and  13 
feet,  or  36  per  cent,  of  the  height,  thick 
at  the  base.  The  concrete  foundation  is 
17  feet  wide,  and  6  feet  deep  below  the 


91 


bottom  of  the  dock,  and  the  fluid  press- 
ure required  for  overturning  would  be 
about  45  Ibs.  per  cubic  foot.  A  coefficient 
of  friction  of  less  than  £  would  be  suffi- 
cient to  guard  against  sliding  under  this 
pressure,  but  owing  to  the  existence  of  a 
thin  seam  of  soft  greasy  silt  between  the 
hard  strata  of  blue  clay  upon  which  the 
foundations  rested,  several  portions  of 
the  wall  slid  forward.  The  original  ground 
level  was  about  15  feet  below  the  top 
of  the  dock  wall,  and  the  excavation  stood 
fairly  as  a  slope  of  1  to  1.  Favorable 
material  for  backing  did  not  appear  to  be 
available. 

The  fact  that  the  stability  of  a  dock 
wall  depends  far  more  upon  the  founda- 
tion than  upon  the  thickness  or  mass  of 
the  wall  itself,  is  well  illustrated  by  the 
quay  wall  at  Carlingford  (Ex.  48,  Fig. 
24).  With  a  height  of  no  less  than  47 
feet*  6  inches,  the  thickness  of  wall  and 
width  of  foundation  at  the  base  are  each 
but  15  feet,  or  less  than  32  per  cent,  of 
the  height,  and  the  mean  thickness  is 
but  24  per  cent.  A  lateral  pressure  of 


92 

half  that  due  to  a  hydrostatic  pressure 
would  probably  suffice  to  overturn  this 
structure. 

In  contrast  with  the  preceding  wall 
may  be  cited  that  of  the  dock  basin  at 
Marseilles  (Ex.  49,  Fig.  25).  In  both  in- 
stances the  foundation  was  good,  and 
the  wall  rested  immediately  upon  it  with- 
out the  interposition  of  any  broad  mass 
of  concrete ; ,  but  the  French  engineer, 
though  the  wall  was  but  32  feet  high, 
made  the  thickness  at  the  base  no  less 
than  16  feet  9  inches,  or  52  per  cent,  of 
the  height— an  unusually  large  propor- 
tion, which  he  was  led  to  adopt  in  conse- 
quence of  the  stratification  of  the 
ground  inclining  towards  the  wall. 

Perhaps  one  of  the  boldest  and  most 
successful  examples  of  a  lightly-propor- 
tioned wharf  wall  is  that  built  by  Colo- 
nel Michon  in  1857  on  the  Moselle  at 
Toul  (Ex.  50,  Figs.  26  and  27).  With  a 
height  of  26  feet,  and  a  batter  of  1  in 
20,  the  thickness  of  the  wall  through  the 
counterforts  is  but  3  feet  7  inches  at  the 
base,  and  though  the  filling  is  ordinary 


93 


*  >f 


u. 


-4 


94 

material,  having  a  slope  of  repose  of  1-J- 
to  1,  and  the  floods  rise  within  6  feet  of 
the  top  of  the  coping,  no  movement 
whatever  has  occurred  since  the  wall  was 
built. 

As  striking  a  contrast  as  could  be 
wished  to  the  above  light  construction  is 
found  in  Sir  John  Macneill's  quay  wall 
at  Grangemouth  harbor  (Ex.  51,  Fig.  28). 
Both  walls  are  of  about  the  same  height, 
but  whilst  the  mean  thickness  of  the 
first  is  only  3.7  feet,  or  \  of  the  height, 
that  of  the  second,  inclusive  of  the  mass 
of  concrete  backing,  is  no  less  than  23 
feet,  or,  say,  f  of  the  height. 

One  of  the  most  troublesome  cases  of 
dock- wall  failures  was  that  at  the  Belfast 
harbor*  (Ex.  52,  Fig.  29).  This  wall 
was  founded  upon  round  larch  piles  15 
feet  long,  10  inches  in  diameter  at  the 
top,  and  4  feet  6  inches  apart  from  cen- 
ter to  center.  Symptoms  of  settlement 
became  apparent  soon  after  the  filling 
was  commenced,  and  some  remedial 

*  Vide  Minutes  of  Proceedings  Inst.  C.E.,  vol.  lv.,  p. 
31. 


95 


96 

measures  were  attempted.  The  ground, 
however,  was  hopelessly  bad,  the  slope 
of  repose  ranging  from  3  to  1  to  6  to  1, 
and  the  backing  material  being  equally 
bad,  the  light  piling  was  inadequate  to 
resist  the  thrust.  Two  years  after  erec- 
tion a  length  of  about  70  lineal  yards  of 
wall  was  overturned  and  carried  forward 
into  the  middle  of  the  dock  entrance,  the 
piles  being  sheared  off  about  6  feet  be- 
low the  bottom  of  the  wall.  The  height 
from  the  top  of  the  pile  to  the  coping  is 
31  feet  6  inches,  and  the  thickness  at  the 
base  16  feet,  or  half  the  height.  On 
good  ground,  therefore,  the  wall  would 
have  had  an  ample  margin  for  stability. 

A  somewhat  similar  failure  occurred  in 
the  instance  of  the  original  side  walls  of 
the  lock  chamber  of  the  Victoria  docks* 
(Ex.  53,  Fig.  30).  These  docks  were  built 
at  a  time  when  little  confidence  was  placed 
in  concrete  as  a  durable  material  for 
dock  work,  and  consequently  the  walls 
were  faced  with  cast-iron  piling  and 
plates,  as  in  previous  instances  at  Black- 

*lbid.,  vol.  xviii.,  p.  462. 


97 

wall  and  elsewhere.  The  foundations 
were  on  a  layer  of  gravel  overlying  the 
clay,  but  the  face  piling  had  little  hold 
in  the  gravel,  and  the  base  of  the  wall 
itself  was  only  some  30  per  cent,  of  the 
height,  hence,  when  the  water  was  let 
into  the  dock,  the  hydrostatic  pressure 
at  the  back  of  the  lock  wall  forced  it 
bodily  forward  into  the  lock,  ploughing 
up  the  puddle  in  front  of  it,  and  break- 
ing tie  bolts  and  tie  piles  as  it  advanced. 
In  reconstruction  a  solid  concrete  wall  20 
feet  thick,  and  having  nearly  treble  the 
stability,  was  carried  through  the  gravel 
down  to  the  clay. 

The  wall  of  the  Victoria  Dock  Exten- 
sion Works,  by  Mr.  A.  M.  Eendel,  M. 
Inst.  C.E.  (Ex.  54,  Fig.  31),  has  a  thick- 
ness of  about  50  per  cent,  of  the  height 
at  the  point  where  the  18 -feet  wide 
foundation  meets  what  may  be  termed 
the  body  of  the  wall,  and  the  wharf 
wall  of  Mr.  Fowler's  Millwall  dock  (Ex. 
55,  Fig.  32)  has  a  maximum  thickness 
of  13  feet  6  inches  for  a  height  of  28 
from  bottom  of  dock  to  coping,  of 


98 

practically  the  same  ratio.  Either  or 
these  walls  would  be  capable  of  resist 
ing  the  full  hydrostatic  pressure. 

An  early  example  of  a  successful  wall 
on  a  very  bad  foundation  is  afforded  by 


Fig.32 


Sir  John  Kennie's  Sheerness  wall  (Ex. 
56,  Fig.  33)  The  subsoil  consisted  of 
loose  running  silt  for  a  depth  of  about  50 
feet,  covered  with  soft  alluvial  mud,  and 
the  depth  at  low  water  was  at  some 
points  as  much  as  30  feet.  A  piled  plat- 
form about  42  feet  in  width,  with  sheet- 


OFTHE 


99 


ing  piles  on  the  river  face,  and  12-inch 
piles  pitched  from  3  to  4  feet  apart  over 
the  whole  area,  and  driven  until  a  15-cwt. 
monkey  falling  25  feet  did  not  move 

Fig.33 


them  more  than  ^  inch  at  a  blow,  was 
prepared,  and  upon  this  the  wall,  no  less 
than  50  feet  in  extreme  height  and  32 
feet  in  effective  thickness  at  the  base, 
was  raised.  In  no  case  has  any  yielding 


100 

or  unequal  settlement  taken  place,  ex- 
cept in  the  instance  of  the  basin  wall, 
the  cracks  in  which  Sir  John  Kennie  at- 
tributed to  other  causes  than  a  failure  in 
the  foundation.  Although  the  voids  in 
the  masonry  were  designedly  filled  in 
with  grouted  chalk  and  other  light  mate- 
rial, the  Sheerness  river  wall  has  per- 
haps a  greater  moment  of  stability  than 
any  other  wall  in  the  world. 

Another  exceptionally  heavy  wall,  more 
than  a  half  century  younger  than  the 
preceding,  is  that  of  the  Chatham  Dock- 
yard Extension  (Ex.  57,  Fig.  34).  The 
height  from  the  bottom  of  the  dock  to 
the  coping  is  39  feet,  and  the  founda- 
tions are  carried  down  to  the  loam  gravel 
or  chalk  at  a  depth  of  4  feet  6  inches 
below  the  bottom  of  the  dock.  The 
thickness  of  the  wall  is  21  feet  at  the 
base,  or,  say,  J-  of  the  extreme  height. 
On  a  hard  chalk  bottom  it  would  resist  a 
fluid  pressure  of  about  80  Ibs  per  cubic 
foot. 

Two  examples  of  Liverpool  dock 
walls,  namely,  that  at  the  Canada  half- 


101 
tide  basin,  and  that  at  the  Herculanean 


docks,  are  given  in  Figs.  35  and  36.    The 


102 

former  (Ex.  58)  is  43  feet  in  extreme 
height,  and  19  feet,  or  44  per  cent,  wide 
at  the  base.  The  latter  (Ex.  59)  is  39 
feet  high,  and  18  feet,  or  46  per  cent, 
wide  at  the  footings,  which  rest  on  a 
marl  bottom.  A  dock  wall  at  Spezzia 
(Ex.  60)  of  somewhat  similar  propor- 
tions, the  height  being  41  feet,  and  the 
width  at  the  bottom  of  foundations  23 
feet,  or  56  per  cent,  of  the  height  is 
shown  on  Fig.  37. 

Walls  made  of  large  concrete  blocks, 
resting  upon  a  mound  of  rubble,  have 
been  constructed  in  many  of  the  Medi- 
terranean ports,  generally  with  success, 
but  occasionally  with  failure,  as  at  Smyr- 
na, where,  owing  to  the  great  settlement, 
six  and  seven  tiers  of  blocks  had  to  be 
superimposed  instead  of  four,  as  in- 
tended, and  the  quay  wall  had  after  all 
to  be  supported  by  a  slope  of  rock  in 
front  extending  up  to  within  7  feet  of 
mean  sea  level,  and  seriously  interfering 
with  the  use  of  the  quays.  The  propor- 
tions arrived  at  by  experience  are  a  width 
of  9  meters  at  the  top,  and  a  thickness 


103 


S 


104 

of  not  less  than  2  meters  for  the  rubble 
mound ;  a  depth  of  7  meters  below  the 
water  line,  and  a  thickness  of  4  meters 
for  the  concrete  block  wall  resting  on 
the  mound  ;  and  a  minimum  thickness  of 
2.5  meters,  and  a  height  of  2.4  meters 
for  the  masonry  wall  coping  the  concrete 
blocks. 

At  Marseilles  (Ex.  61,  Fig.  38),  the 
top  of  the  rubble  mound  is  only  6 
meters  below  the  water-line,  so  vessels 
occasionally  bump ;  and  the  concrete 
block  wall  3.4  meters,  or  40  per  cent,  of 
the  height,  in  thickness  has  proved 
rather  less  stable  under  the  contingencies 
of  working  and  the  surcharge  of  build- 
ings and  goods  than  is  considered  desir- 
able. 

Examples  are  not  wanting,  however, 
of  walls  founded  on  rubble  mounds 
where  the  thickness  holds  a  smaller  ratio 
to  the  height  than  the  42  per  cent.,  con- 
sidered necessary  by  the  French  engi- 
neers. Mr.  Fowler  has  made  concrete 
block  walls  in  the  Rosslare  Harbor  (Ex. 
62)  42  per  cent,  of  the  height  on  the  sea 


105 

face,  and  but  28  per  cent,  on  the  harbor 
side,  but  cross  walls  at  50-feet  intervals 
considerably  strengthen  the  work.  The 
inner  wharf  wall  of  the  Holyhead  new 
harbor,  again  (Ex.  63,  Fig.  39),  is  27  feet 
high  and  8  feet  thick,  a  ratio  of  under  30 
per  cent.,  but  though  stable,  the  line  of 
coping  is  somewhat  wavy  on  plan.  The 
original  wall  of  the  West  Pier  at  White- 
haven  (Ex.  64,  Fig.  40),  is  42  feet  6  inches 
high,  with  a  thickness  of  8  feet  6  inches 
between  the  buttresses,  which  latter  are 
6  feet  deep  by  about  4  feet  wide  and  15 
feet  apart ;  but  the  lightest  of  all,  per- 
haps, is  the  dry  masonry  outer  wall  of  the 
St.  Katherine's  breakwater,  Jersey,  (Ex. 
65,  Fig.  41),  which  is  only  14  feet  wide 
at  the  base  for  a  total  height  of  50  feet, 
or  a  ratio  of  28  per  cent. 

It  must  not  be  forgotten,  of  course, 
that  the  three  latter  walls  have  to  sup- 
port rubble  hearting  only,  instead  of 
sand  and  other  material,  having  a  much 
flatter  slope  of  repose.  Occasionally,  as 
has  been  stated  (Ex.  22),  rubble  will  not 
stand  at  less  than  1£  to  1 ;  but  at  Holy- 


106 

head  and  Alderney  the  slope  of  the  rub- 
ble mound  on  the  harbor  side  is  only 
about  1J  to  1.  At  Cherbourg  it  is  1  to 
1,  and  at  Leghorn  the  large  concrete 
blocks  are  found  to  be  stable  at  a  slope 


Fig.40 


Fig,4l 


of  f  to  1.  By  a  very  little  care  in  selec- 
tion, the  thrust  of  a  rubble  filling  may 
be  reduced  to  a  fraction  of  that  arising 
from  bad  material,  and  indeed  in  the 
ordinary  run  of  fishing  piers  in  the  North 


107 

of  Scotland,  however  great  the  height, 
the  face  wall  of  the  rubble-hearted  pier 
consists  simply  of  stones  from  3  to  4 
feet  in  depth,  laid  dry  to  a  batter  of  about 
1  in  5.  The  north-east  pier  at  Seham, 
again,  has  an  inner  wall  25  feet  high* 
battering  1^  inch  to  the  foot,  and  only  5 
feet  thick,  and  many  similar  examples 
are  to  be  found  at  other  points  of  the 
coast. 

The  most  cursory  examination  of  cases 
of  failure  cited  above  will  serve  to  justify 
the  statement  that  the  numerous  dock- 
wall  failures  do  not  afford  any  direct 
evidence  as  to  the  actual  lateral  pressure 
of  earthwork.  Thus,  remembering  Gene- 
ral Burgoyne's  battering  wall,  only  17 
per  cent,  of  the  height  in  thickness, 
supported  the  heavy  sodden  filling  at  its 
back,  no  calculation  is  required  to  show 
that  the  32  and  45  per  cent.  Southamp- 
ton Dock  counterfeited  wall,  the  42  per 
cent.  Avonmouth  Dock  wall,  the  36  per 
cent.  West  India  Dock  wall,  the  50  per 
cent.  Belfast  Harbor  wall,  and  the  30  per 
cent.  Victoria  Dock  wall,  would  all  have 


103 

stood  perfectly  had  the  foundation  been 
rock,  as  in  the  instances  of  General  Bur- 
goyne's  experimental  walls,  instead  of 
the  mud,  clay,  and  silt  which  it  actually 
was. 

Not  only  the  strength,  but  the  type  of 
cross-section,  is  singularly  indicative  of 
the  small  influence  which  theory  and  ex- 
periment have  exercised  upon  the  design 
of  dock  walls.  If  the  early  theorists  and 
experimentalists  were  in  accord  upon  one 
point,  it  was  upon  the  immense  advant- 
age afforded  by  a  court terforted  wall. 
Lieutenant  Hope  was  led  by  his  experi- 
ments to  conclude  that  if  good  counter- 
forts were  introduced,  the  merest  skin  of 
face  wall  would  suffice  for  the  portion 
between  them,  and  theorists  of  course 
arrived  at  the  same  conclusion,  from  a 
comparison  of  the  moments  of  stability 
of  rectangular  blocks  of  masonry  edge- 
wise and  flatwise.  Nevertheless,  in  only 
one  of  the  preceding  dock  walls,  and  that 
one  forty  years  old,  are  counterforts  in- 
troduced. In  practice  it  was  found  that 
counterforts  frequently  separated  from 


109 

the  body  of  the  wall,  and  they  were  con- 
sequently   regarded    as    untrustworthy. 
It  is  open  to  question  whether  this  con- 
clusion does  not  require  reconsideration 
in    these    days   of    cheap,    strong,    and 
easily-moulded  Portland  cement  concrete. 
Nothing  but  blasting  would  separate  the 
counterforts   from  a  good  concrete  wall. 
The  author  has  used  concrete  in  many 
varieties  of  structures,  and  as  long  back 
as  fifteen  years   built  a  four-story  ware- 
house, walls  and  floors,  entirely  of  con- 
crete, without  the  introduction  of    any 
iron    girders.     He   is   bound   to    admit, 
however,  that   by   far   the   boldest  and 
most    thorough  adaptation    of    the  ma- 
terial to  multifarious  uses  met  with  by 
him  was  in    the  instance  of  some   farm 
buildings  in  an  out-of-the-way  district  in 
Co.  Kerry,  Ireland.       The  small  tenant- 
farmer  and  his  laborers — none  of  whom 
were   receiving   over  11s.  a  week — with-  . 
out  skilled  assistance    of  any  kind,  had 
constructed  dwelling-house,  cattle-sheds, 
and  hay -barn  wholly  of  concrete.       The 
cattle-shed   was    roofed    with    concrete 


110 

arches  of  15 -feet  span,  1  foot  rise,  and 
4  inches  thick,  springing  from  octagonal 
concrete  pillars  8  inches  in  diameter, 
spaced  15  feet  apart  from  center  to  center. 
A  layer  of  concrete  constituted  the  pav- 
ing, concrete  slabs  divided  the  stalls,  the 
cattle  fed  and  drank  out  of  concrete 
troughs,  the  windows  were  glazed  in 
concrete  mullions,  the  gates  hung  on  con- 
crete posts,  and  the  farmer  seemed  to 
regret  somewhat  that  he  had  not  adopted 
concrete  doors  and  concrete  five-bar 
gates. 

Portland  cement  concrete  being  thus 
possessed  of  such  great  tenacity,  there  is 
no  risk  of  counterforts  separating  from 
the  body  of  a  wall,  but  it  by  no  means 
follows  that  there  would  be  any  advant- 
age in  using  them  in  other  than  excep- 
tional cases.  In  practice,  as  failures  have 
shown,  it  is  weight,  with  the  consequent 
grip  on  the  ground,  rather  than  a  high 
moment  of  stability,  that  is  required  iri  a 
dock  wall.  It  may  be  asked,  with  reason 
why  a  bad  bottom  should  affect  the 
thickness  of  a  retaining  wall,  or,  in  other 


Ill 

words,  why  the  foundation  should  not 
first  be  made  good,  and  then  a  wall  of 
ordinary  thickness  be  built  upon  it.  The 
answer,  of  course,  is  that  if  weight  is  re- 
quired to  prevent  sliding,  it  is  just  as 
economical  to  distribute  the  material 
over  the  general  body  of  the  wall  as  to 
confine  it  to  the  foundations.  It  follows, 
therefore,  that  under  the  stated  condi- 
tions the  adoption  of  a  counterforted 
wall  would  lead  to  no  economy  in  ma- 
terial, whilst  it  would  involve  additional 
labor  in  construction. 

A  dock  wall  is  subject  to  far  larger 
contingencies  than  an  ordinary  retaining 
wall,  and  the  required  strength  will  be 
included  only  within  correspondingly 
large  limits.  Hydrostatic  pressure  alone 
may  more  than  double  or  halve  the 
factor  of  safety  in  a  given  wall.  Thus, 
with  a  well-puddled  dock  bottom,  the 
subsoil  water  in  the  ground  at  the  back . 
of  the  walls  will  frequently  stand  far 
below  the  level  of  the  water  in  the  dock, 
and  the  hydrostatic  pressure  may  thus 
wholly  neutralize  the  lateral  thrust  of  the 


112 

earth,  or  even  reverse  it,  as  in  the  case  of 
the  inner  retaining  walls  on  the  Soon- 
kesala  canal,  some  of  which,  though  35 
feet  in  height,  are  only  2  feet  thick  at  the 
top  and  7  feet  6  inches  at  the  base.  On 
the  other  hand,  with  a  porous  subsoil  at 
a  lock  entrance,  the  back  of  the  walls 
may  be  subject,  on  a  receding  tide,  to  the 
full  hydrostatic  pressure  due  to  the 
range  of  that  tide  plus  the  lateral  press- 
ure of  the  filling.  Again,  the  water  may 
stand  at  the  same  level  on  both  sides  of 
the  wall,  but  may  or  may  not  get  under- 
neath it.  If  the  wall  is  founded  on  a 
rock  or  good  clay,  there  is  no  more 
reason  why  the  water  should  get  under 
the  wall  than  that  it  should  creep  through 
any  stratum  of  a  well-constructed  ma- 
sonry or  puddle  dam,  and  under  those 
ci  -cum stances  the  presence  of  the  water 
will  increase  the  stability  by  diminishing 
the  lateral  thrust  of  the  filling.  With 
rubble  filling,  assuming  the  weight  of 
the  solid  stone  to  be  155  Ibs.  per  cubic 
foot,  and  the  voids  to  be  35  per  cent., 
the  \veight  of  the  filling  would  be  100 Ibs. 


113 

per  cubic  foot  in  air,  and  59  Ibs  in  water, 
and  the  lateral  thrust  will  be  that  due  to 
the  latter  weight. 

If,  however,  as  is  perhaps  more  fre- 
quently the  case,  the  wall  is  founded  on 
a  porous  stratum,  the  full  hydrostatic 
pressure  will  act  on  the  base  of  the  wall, 
and  reduce  its  stability  in  practical  cases 
by  about  one -half.  Thus,  the  30 -ton 
concrete  block  walls  on  rubble  mounds, 
at  Marseilles  and  elsewhere,  have  the 
stability  due  to  a  weight  of,  say,  130  Ibs. 
per  cubic  foot  in  the  air,  and  66  Ibs.  per 
cubic  foot  in  sea  water :  but  the  rubble 
filling  at  the  back  of  the  wall,  being  simi- 
larly immersed,  is  also  reduced  in  weight, 
and  consequently  thrust  to  a  correspond- 
ing extent,  so  the  factor  of  safety  is  un- 
affected. 

In  walls  with  offsets  at  the  back,  as  in 
Figs.  25  and  36,  and  water  on  both  sides, 
the  stability  will  be  much  increased  by 
the  hydrostatic  pressure  on  the  top  of ' 
the  offsets,  should  the  wall  rest  on  an 
impermeable  foundation.  It  is  generally 


114 

assumed,  in  theoretical  investigations,* 
that  the  weight  of  earthwork  super-im- 
posed vertically  over  the  offsets  should 
be'included  in  the  weight  of  the  wall  in 
estimating  the  moment  of  stability ;  but 
the  author  has  found  no  justification  in 
practice  for  this  assumption.  He  has  in- 
variably observed  that  when  a  retaining 
wall  moves  by  settlement  or  otherwise, 
it  drops  away  from  the  filling,  and  cavi- 
ties are  formed.  A  settlement  of  but  -fa 
of  an  inch,  after  the  backing  had  become 
thoroughly  consolidated,  would  suffice  to 
relieve  the  offsets  of  all  vertical  pressure 
from  the  superimposed  earth,  and  the 
latter  cannot  therefore  be  properly  con- 
sidered as  contributing  to  the  moment  of 
stability. 

A  wall  with  deep  offsets  at  the  back  is 
not  a  desirable  form  where  the  foundation 
is  bad,  and  where,  consequently,  the 
pressure  over  the  foundation  should  be 
as  uniform  as  possible,  so  that  a  settle- 
ment may  take  the  form  of  a  uniform 


*  Vide  "  A  Manual  of  Civil  Engineering."     By  W.  J. 
M.  Rankine,  p.  402. 


115 

sinking,  and  not  a  tilting  forward  of  the 
coping  by  reason  of  the  toe  sinking  faster 
than  the  back  of  the  wall.  A  paneled 
wall,  such  as  that  shown  on  Figs.  11  to 
14,  though  not  admissible  in  dockwork, 
is  on  bad  ground  far  less  liable  to  come 
over  than  a  wall  with  offsets  at  the  back, 
and  with  a  consequent  concentration  of 
weight  at  the  front,  where  the  conditions 
of  a  lateral  thrust  especially  require  that 
it  should  not  be. 

The  latter  conditions  also  indicate  the 
expediency  of  adopting  raking  piles,  as  in 
Fig.  33,  rather  than  vertical  piles,  as  in 
Fig.  29,  where  a  piled  foundation  is  un- 
avoidable- Thus,  taking  an  ordinary 
case  of  dock  wall,  in  which  the  factor  of 
safety,  as  regards  overturning,  is  3,  and 
the  ratio  of  weight  of  wall  to  the  lateral 
pressure  of  earthwork  required  to  over- 
turn it  is  If  to  1,  it  follows  that  if  the 
foundation  piles  are  driven  at  the 
rate  of  1  to  3  +  If  =  1 :  5  there  will  be 
no  transverse  strain  tending  to  break 
them  off,  as  in  the  case  illustrated  by  Fig. 
29,  and  no  tendency  to  plough  up  the 


116 

soft  ground   in  front  of  the  toe  of  the 
wall. 

If  an  engineer  could  tell  by  inspection 
the  supporting  power  and  frictional  ad- 
hesion of  every  bit   of  soil  laid  bare,  or 
see  through  5  or  10  feet  of  earth  into  a 
"  pot   hole,"   or  .layer  of  slimy    silt,    he 
might  avoid  many  failures,  and  even  hope 
to  frame  some  useful  equations  for  ob- 
taining the  required  thickness  of  a  dock 
wall.     Taking  things  as   they  are,  how- 
ever, it  is  hardly  worth  while  to  use  even 
a  scale  and  compass   in  such  work,  for 
being  in   possession  of  all  the  informa- 
tion obtainable  about  the  foundation  and 
backing,  an  engineer  may  at  once  sketch 
as  suitable  a  cross -section  for  the  parti- 
cular case  as  he  could  hope  to  arrive  at 
after  any  amount  of  mathematical  inves- 
tigation.    Something  must   be  assumed 
in  any  event,    and   it   is  far  more  simple 
and  direct  to  assume  at  once   the   thick  • 
ness  of  the  wall  than  to  derive  the  latter 
from  equations  based  upon  a  number  of 
uncertain  assumptions  as  to  the  bearing 
power  of  the  foundations,  the  resistance 


117 

to  gliding,  and  other  elements.  This 
being  so,  it  has  often  struck  the  author 
that  the  numerous  published  tables 
giving  the  calculated  required  thicknesses 
of  retaining  walls  to  three  places  of  deci- 
mals, stand  really  on  exactly  the  same 
scientific  basis,  and  have  the  same  prac- 
tical value,  as  the  weather  forecasts  for 
the  year  in  Old  Moore's  Almanack.  In 
both  cases  a  pretence  is  made  of  foretell- 
ing what  experience  has  shown  can  often 
not  be  known  until  after  the  event.  One 
well-known  authority  gives  young  en- 
gineers the  choice  of  five  hundred  and 
forty-four  different  thicknesses  for  a 
simple  vertical  rectangular  retaining 
wall,  so  that  an  unfortunate  neophyte 
might  not  unreasonably  conclude  that 
the  task  before  him  was  not  to  decide 
whether,  say,  a  32 -feet  wall  should  be  20 
feet  thick,  as  in  Example  60,  or  9  feet,  as 
in  Example  62,  but  whether  it  should  be 
14  feet  6  inches  or  14  feet  5J  inches 
thick. 

Although  dock   wall   failures   do   not 
afford   any  data  as  to  the  actual  lateral 


118 

pressure  of  earthwork,  a  knowledge  of 
the  latter  will  enable  much  valuable  in- 
formation to  be  deduced  as  to  the  bearing 
power  of  soil  and  other  matters  from 
such  failures,  and  the  data  so  obtained 
will  be  applicable  to  other  structures 
beside  retaining  walls.  Knowing  the 
actual  lateral  thrust,  the  coefficient  of 
friction  of  the  base  of  a  wall  which  has 
been  pushed  forward  on  the  ground  can 
be  at  once  deduced,  but  if  the  theoretical 
as  distinguished  from  the  actual  thrust 
were  introduced  into  the  equation,  the 
result  would  be  valueless. 

The  aim  of  the  author  in  the  present 
paper  has  been  to  set  forth  as  briefly  as 
possible  what  he  knows  regarding  the 
actual  lateral  thrust  of  different  kinds  of 
soil,  in  the  hope  that  other  engineers 
would  do  the  same,  and  that  the  infor- 
mation asked  for  by  Professor  Barlow 
more  than  half  a  century  ago  may  be  at 
last  obtained.  Although  the  acquirement 
of  the  missing  data  would  probably  lead 
to  no  modification  in  the  general  propor- 
tions of  retaining  structures,  since  these 


119 

are  based  upon  dearly  bought  experience, 
it  is  none  the  less  desirable  that  it  should 
be  obtained  ;  for  an  engineer  should  be 
able  to  show  why  he  believes  that  a  given 
wall  will  stand  or  fall.  To  assume  upon 
theoretical  grounds  a  lateral  thrust, 
which  experiments  prove  to  be  excessive, 
and  to  compensate  for  this  by  giving  no 
factor  of  safety  to  the  wall,  is  not  a  scien- 
tific mode  of  procedure. 

Experience  has  shown  that  a  wall  £  of 
the  height  in  thickness,  and  battering  1 
inch  or  2  inches  per  foot  on  the  face, 
possesses  sufficient  stability  when  the 
backing  and  foundation  are  both  favor- 
able. The  author,  however,  would  not 
seek  to  justify  this  proportion  by  assum- 
ing the  slope  of  repose  to  be  about  1  to 
1,  when  it  is  perhaps  more  nearly  1^  to  1, 
and  a  factor  of  safety  to  be  unnecessary, 
but  would  rather  say  that  experiment 
has  shown  the  actual  lateral  thrust  of 
good  filling  to  be  equivalent  to  that  of  a 
fluid  weighing  about  10  Ibs.  per  cubic 
foot,  and  allowing  for  variations  in  the 
ground,  vibration,  and  contingencies,  a 


120 

factor  of  safety  of  2,  the  wall  should  be 
able  to  sustain  at  least  20  Ibs.  fluid  press- 
ure, which  will  be  the  case  if  J  of  the 
height  in  thickness. 

It  has  been  similarly  proved  by  expe- 
rience that  under  no  ordinary  conditions 
of  surcharge  or  heavy  backing  is  it  ne- 
cessary to  make  a  retaining  wall  on  a 
solid  foundation  more  than  double  the 
above,  or  \  of  the  height  in  thickness. 
Within  these  limits  the  engineer  must 
vary  the  strength  in  accordance  with  the 
conditions  affecting  the  particular  case. 
Outside  these  limits  the  structure  ceases 
to  be  a  retaining  wall  in  the  ordinary  ac- 
ceptation of  the  term.  A  9 -inch  brick 
facing  might  secure  the  face  of  a  friable 
chalk  cutting  which,  if  suffered  to  re- 
main exposed  to  the  action  of  the  weather, 
would  crumble  down  to  a  slope  of  1  to  1, 
and  a  massive  bridge  pier,  with  an  "ice- 
breaker "  cutwater,  might  stand  firm 
against  an  avalanche,  but  in  neither  case 
could  the  structure  be  fairly  stated  to  be 
a  retaining  wall. 

Hundreds  of    revetments    have    been 


121 

built  by  Royal  Engineer  officers  in  ac- 
cordance with  General  Fanshawe's  rule 
of  some  fifty  years  ago,  which  was  to 
make  the  thickness  of  a  rectangular  brick 
wall,  retaining  ordinary  material,  24  per 
cent,  of  the  height  for  a  batter  of  ^, 
25  per  cent  for  |,  26  per  cent,  for  £,  27 
per  cent,  for  -fa,  28  per  cent,  for  -^  30 
per  cent,  for  .-fa ,  and  32  per  cent,  for  a 
vertical  wall. 

As  a  result  of  his  own  experience  the 
author  makes  the  thickness  of  retaining 
walls  in  ground  of  an  average  character 
equal  to  J  of  the  height  from  the  top  of 
the  footings,  and  if  any  material  is  taken 
out  to  form  a  face  panel,  three-fourths  elf 
it  are  put  back  in  the  form  of  a  pilaster. 
The  object  of  the  panel,  as  of  the  1^ 
inch  to  the  foot  batter  which  he  gives  to 
the  wall,  is  not  to  save  material,  for  this 
involves  loss  of  weight  and  grip  on  the 
ground,  but  to  effect  a  better  distribu 
tion  of  pressure  on  the  foundation.  It 
may  be  mentioned  that  the  whole  of  thp 
walls  on  the  District  railway  were  de- 
signed on  this  basis,  and  JiaLJJifire  has 


122 

not  been  a  single  instance  of  settlement, 
or  of  coming  over  or  sliding  forward. 
The  author  has  in  the  present  paper 
analyzed  a  few  dozen  experiments,  and 
discussed  as  many  more  facts;  but  an  en- 
gineer's experience  is  the  outcome  not  of 
a  few  facts,  but  of  the  thousands  of  in. 
cidents  which  force  themselves  on  his  at- 
tention in  carrying  out  work,  and  it  is  this 
experience,  acquired  in  the  construction 
of  works  of  a  somewhat  special  character, 
which  has  convinced  the  author  that  the 
laws  governing  the  lateral  pressure  of 
earthwork  are  not  at  present  satisfac- 
torily formulated. 


DISCUSSION. 

Mr.  B.  BAKEK  desired  to  add,  that  his 
object  in  bringing  forward  the  paper  was 
not  so  much  to  present  certain  facts  for 
criticism  as  to  induce  others  to  give  the( 
results  of  their  experience,  and  if  every 
one  helped  a  little  he  thought  a  very  use- 
ful result  would  be  attained. 

Mr.  W.  AIRY  said  he  had  given  consid- 


123 

erable  thought  and  attention  to  the  sub- 
ject of  earthwork,  and  he  considered  the 
collection  of  examples  in  the  paper 
would  make  it  an  extremely  useful  one 
for  purposes  of  reference.  The  subject 
of  earthwork  was  a  very  difficult  one  to 
deal  with,  and  he  wished  to  point  out 
briefly  in  what  this  difficulty  consisted. 
A  B  C  D  (Fig.  42)  might  be  taken  to  be 

Fig.42 


the  section  of  some  ground  having  a 
small  vertical  cliff  at  B  C.  There  would 
be  a  tendency  for  the  ground  to  break 
away  and  come  down  along  some  such 
line  as  D  B.  The  whole  problem  of  the 
stability  of  the  ground,  both  as  affecting 
the  slope  of  the  earth  and  the  pressure 
against  a  retaining  wall,  depended  upon 
the  accurate  determination  of  the  line  D 


124 

B.  It  was  not  an  exceedingly  difficult 
matter  to  determine  this  line,  if  the  con- 
stants of  cohesion,  friction,  and  weight 
of  the  ground  were  known  ;  and  he  had 
himself  dealt  with  the  problem  in  a 
paper  communicated  to  the  Institution. 
The  mechanical  conditions  of  equi- 
librium were  very  simple ;  the  force  tend- 
ing to  bring  the  earth  down  was  the 
weight  of  it ;  the  forces  tending  to  keep 
it  from  coming  down  were  the  friction 
along  the  line  D  B  and  the  cohesion  of 
the  ground  along  that  line.  All  those 
forces  acted  according  to  well-under- 
stood laws,  and  therefore  if  the  con- 
stants of  weight,  cohesion,  and  friction 
of  any  particular  ground  were  known,  it 
was  not  difficult  to  find  out  the  exact 
position  of  the  line  D  B,  and  therefore 
the  pressure  on  the  retaining  wall,  or  the 
shape  of  the  slope.  The  question  then 
arose,  what  was  the  real  difficulty  of  con- 
structing tables  for  practical  use  with  re- 
gard to  earthwork?  Simply  this,  that 
the  varieties  of  ground  were  infinite  in 
number  and  very  wide  in  range,  and 


125 

when  that  was  the  case  it  was  quite  idle 
to  think  of  constructing  tables  for  prac- 
tical use.  A  man  having  a  particular 
kind  of  earth  to  prescribe  for,  would  not 
be  able  to  ascertain  by  inspection  what 
the  constants  of  that  earth  were,  and 
therefore  he  would  not  know  where- 
abouts in  a  table  to  look ;  he  would  have 
to  determine  the  constants  for  himself ; 
and  if  he  had  to  do  that  he  had  to  do 
the  whole  work,  and  the  tables  were  of 
no  use  to  him.  He  thought  the  author 
had  rather  overlooked  the  enormous 
number  of  conditions  of  earth  when  he 
contrasted  the  small  number  of  experi- 
ments upon  earthwork  with  the  large 
number  of  experiments  made  with  tim; 
ber.  A  piece  of  oak  would  give  very 
nearly  the  same  results  for  strength, 
elasticity,  and  so  on,  whether  it  was 
grown  in  Kent  or  in  Yorkshire ;  and, 
therefore,  when  a  few  experiments  had 
been  made  upon  it,  it  was  not  necessary 
to  repeat  them  over  and  over  again. 
That  was  not  the  case  with  earthwork, 
because  the  conditions"  were  so  exceed- 


126 

ingly   variable.     He    exhibited    a    little 
rough  machine  he  had  used  for  testing 
earthwork   and   taking   the  cohesion  of 
the  ground.     The  block  of  wood  might 
be  taken   to   represent   a   block  of   raw 
clay  taken  out  of  a  cutting.     There  was 
a  common  lever  balance,  and  a  couple  of 
movable  cheeks  were   fitted  into  chases 
cut  in  the  sides  of  the  clay  block ;  and 
the  clay  having  been  rammed  in  a  box  so 
that  it  could  not  move,  weights  were  put 
in  the  scale  until  the  head  was  torn  off. 
After    subtracting    the    weight    of    the 
piece  that  was  torn  off,  and  measuring 
the  area  of  the  cross  section   that  was 
broken,   the   constant   of    cohesion   was 
determined.     For   the   constant  of  fric- 
tion he   arranged   a   certain   number  of 
blocks  of  the  same  clay  in  a  tray,  and 
scraped  them  off  smooth;  then  he  had 
another  block  of  clay  with  a  smooth  sur 
face  which  he  put  on  it,  and  then  tilted  the 
tray  until  the  loose  block  slid;  that  gave 
the   coefficient   of   friction.     He   should 
like  to    refer   to    the   exceedingly   wide 
range  of    tenacity   shown    by   different 


127 

kinds  of  clay.  In  one  set  of  experi- 
ments with  ordinary  brick  loam,  that 
clay  gave  a  coefficient  of  cohesion  of  168 
Ibs.  per  square  foot,  and  a  coefficient  of 
friction  of  1.15.  With  some  shaley  clay 
out  of  a  cutting  in  the  Midlands,  he  had 
found  a  coefficient  of  tenacity  of  800 
Ibs.  per  square  foot,  and  a  coefficient 
of  friction  of  0.36.  That  was  a  very 
wide  range,  but  it  was  only  a  part  of 
what  was  actually  to  be  found  in  practice. 
Mr.  L.  F.  YEKNON  HARCOTJRT  wished  to 
say  a  few  words  on  the  subject,  as  the 
author  had  referred  to  two  or  three 
works  with  which  he  had  been  connected. 
The  author  had  pointed  out,  from  the 
experiments  he  had  recorded,  that  the 
pressure  upon  the  back  of  a  retaining 
wall  was  a  good  deal  less  than  it  was 
theoretically  supposed  to  be — about  one- 
half — but  as  he  allowed  a  factor  of  safe- 
ty of  2,  it  apparently  came  to  very  much 
the  same  thing.  With  regard  to  walls 
on  a  rubble  mound,  the  author  remarked 
that  the  base  was  in  many  cases  small. 
That,  he  thought,  was  owing  t3  two 


128 

causes ;  first,  that  with  a  rubble  mound 
for  a  base  there  was  no  chance  of  slid- 
ing ;  and  secondly,  that  in  those  cases 
there  was  a  rubble  filling  behind,  which 
he  supposed  was  about  as  good  a  mate- 
rial for  backing  as  could  be  got.  The 
slope  of  the  inner  face  of  the  rubble 
mound  of  the  breakwater  at  Alderney 
harbor  had  been  referred  to  as  1 J  to  1 ; 
but  it  ought  to  be  remembered  that  in 
that  case  the  materials  used  were  very 
large  blocks  of  stone,  and  therefore  the 
slope  would  be  naturally  steeper  than 
under  more  ordinary  conditions.  Kef- 
erence  had  also  been  made  in  the  paper 
to  St.  Katharine's  breakwater,  Jersey,  as 
an  example  of  a  wall  built  with  a  very 
small  base.  The  author  took  the  whole 
of  the  height  of  that  wall  as  the  proper 
height;  but  it  would  be  observed  that 
the  top  of  the  wall  had  what  used  to  be 
called  a  promenade  along  it,  and  there- 
fore the  whole  of  the  filling  did  not  ap- 
ply to  the  entire  height  of  the  wall. 
The  author  stated  that  the  base  was  28 
per  cent,  of  the  height  of  the  wall,  but 


121) 

leaving  out  the  promenade  it  would  be 
35  per  cent.  Of  course  it  would  be 
something  intermediate,  as  there  would 
only  be  the  small  piece  of  filling  under 
the  promenade  to  be  taken  into  account 
additional,  instead  of  what  would  be  the 
filling  at  the  back  if  it  was  filled  up  en- 
tirely to  the  top  level.  The  author  had 
referred  to  the  West  India  dock  wall, 
and  stated  that  several  portions  of  it  had 
come  forward.  That,  however,  was  not 
quite  the  case.  It  was  true  that  two 
portions  of  the  south  wall  came  forward 
— that  two  surfaces  of  clay  at  some  little 
depth  below  the  wall  slid  upon  one  an- 
other. Probably  some  seam  of  sand  or 
silt  was  washed  out  by  the  water  behind 
the  wall  from  between  two  layers  of  clay, 
and  in  that  way  the  two  detached  sur- 
faces of  clay  were  free  to  slide  upon  one  * 
another.  He  was  quite  certain  of  the 
exact  position  of  the  surfaces  of  rup- 
tmv,  because  he  saw  the  two  surfaces  of 
clay  after  the  excavation  was  made  for  re- 
build ing  the  wall,  and  they  were  as  smooth 
as  glabn.  The  romecly  for  that  appeared 


130 

to  him  to  be  very  simple,  and  it  was  cer- 
tainly successful  in  the  case  in  point. 
The  wall  had  failed,  as  the  author  had 
stated,  not  from  any  fault  in  the  thick- 
ness or  the  weight,  but  simply  owing  to 
the  sliding  forward  ;  and  instead  of  add- 
ing any  further  weight  to  the  wall,  the 
foundations  were  carried  down  to  a 
greater  depth ;  but  it  only  required  2  or 
3  feet  more  in  depth  in  the  basin  wall 
foundations  that  had  to  be  executed  af- 
terwards under  precisely  similar  condi- 
tions. That  was  quite  sufficient  to. keep 
the  wall  in  a  perfect  state  of  equilibrium 
without  the  least  coming  forward;  and 
he  should  imagine  that  was  decidedly 
better  on  the  whole  than  adding  to  the 
weight  of  the  wall.  It  appeared  to  him 
that  practice  was  rather  contrary  to 
theory  in  giving  too  great  a  thickness  to 
the  top  of  the  wall,  and  too  small  a 
thickness,  comparatively  speaking,  to  the 
bottom  ;  and  that  it  would  be  better  to 
have  a  wall  more  of  the  shape  of  the 
Sheerness  wall  a  good  deal  lessened  at 
the  top,  rather  than  a  wall  like  those 


131 

generally  adopted,  which  had  more  par- 
allel faces  with  a  little  additional  thick- 
ness from  the  batter.  He  thought  it 
would  be  better  to  make  a  wall  narrower 
at  the  top  and  widening  out  more  to- 
wards the  bottom,  and  to  bring  the 
foundations  of  the  wall  well  down  into 
the  ground  so  as  to  prevent  any  chance 
of  sliding.  In  the  case  of  the  West 
India  dock  wall,  besides  the  badness  of 
the  backing,  there  was  a  large  amount  of 
water  that  seemed  to  percolate  from  the 
Millwall  docks,  which  were  filled  with 
water  while  the  wall  was  being  built,  the 
docks  not  having  been  puddled.  It  was 
clearly  shown  that  that  had  a  considera- 
ble effect,  because  the  north  wall,  though 
it  was  built  in  exactly  the  same  manner, 
and  though  the  water  of  the  Export 
dock  was  really  nearer,  stood  perfectly, 
as  there  was  not  the  same  amount  of 
water  pressure  at  the  back,  owing  to  the 
water  being  unable  to  penetrate  through 
the  silted-up  bottom  of  the  Export  dock. 
He  considered  that  the  Institution  was 
much  indebted  to  the  author  for  collect- 


132 

ing  and  comparing  so  many  valuable 
facts,  as,  whilst  descriptions  of  particu- 
lar works  were  very  useful,  it  was  by 
taking  a  general  survey,  from  time  to 
time,  of  the  existing  state  of  knowledge, 
in  any  special  branch,  that  definite  prog- 
ress in  engineering  science  was  most 
likely  to  be  promoted. 

Mr.  J.  WOLFE  BAEKY  believed  the  state- 
ment was  true,  that  the  pressure  against 
retaining  walls  did  not  approach  to  the 
theoretical  thrust ;  at  the  same  time  he 
was  of  opinion  that  large  retaining  walls 
gave  the  engineer  as  much  anxiety  as  any 
work  he  ever  undertook.  It  should  be  re- 
membered that,  as  a  rule,  the  thrust  which 
the  walls  had  to  bear  came  against  them 
when  the  material  of  which  they  were 
composed  was  green,  and  unless  con- 
tractors and  others  were  very  careful  in 
strutting  the  new  work,  and  allowing 
plenty  of  time  for  the  material  to  set, 
there  would  be  a  condition  of  affairs  in  the 
early  stages  of  the  wall  which  would 
never  arise  after  the  materials  were  thor- 
oughly consolidated.  He  wished  to  point 


133 

out  that  it  was  for  such  reasons  most  en- 
gineers were  now  getting  to  realize  the 
extreme  desirability  of  using  cement 
as  much  as  possible.  The  early  stages 
of  engineering  works  were  generally 
those  in  which  the  greatest  risks  were 
run,  and  if  a  slow-setting  material 
were  used,  the  strains  would  be  exerted 
against  it  in  its  weakest  condition,  and 
disasters  would  occur  such  as  would  not 
happen  at  a  later  period.  He  agreed 
with  the  statement  of  the  author  with 
regard  to  the  failure  of  retaining  walls. 
No  doubt,  in  ninety  cases  out  of  a  hun- 
dred, the  failure  happened  from  bad 
foundations.  The  remedy  in  railway 
works  was  in  many  cases  that  shown  in 
Fig.  17,  which  practically  amounted  to 
strutting  the  toe  of  the  wall  against  the 
.opposite  wall,  and  so  preventing  it  slid- 
ing forward.  That  was  a  very  simple 
arrangement,  and  resembled  in  its  effect 
the  strutting  of  timber,  which  was  gen- 
erally carried  out  as  a  temporary  meas- 
ure by  a  contractor,  when,  an  invert  was 
going  to  be  put  in.  If  the  engineer 


,    -:.,  -    .    --.-::  -:::.:  -r 


-..__..^ _.  .. 

_    .    ...     ••-'.'          •  .._..         .  _  :.    . 


^~..   1  --•  .     V     '.   : 

giuond  rmQwmr 


Of    8UM    Of 

cencdin  It:  and  HWT 
for  Ike  foDnegs  uid  ' 


Wt 


latter  pax*  he  slated  that  hi 
had  ptelly  wall   accorded    ittlli 


I3G 

For  instance,  he  gave  the  theoretical 
thickness  for  a  retaining  wall  in  ground 
that  naturally  stood  at  a  slope  of  1^  to 
1  as  31  per  cent,  of  the  height ;  and  in 
the  last  paragraph  but  one  he  said  his 
habit  had  been  to  make  his  walls  ^,  or  33 
per  cent.;  and  in  the  Table  with  slopes 
from  1  to  1  to  4  to  1,  which  included  all 
that  engineers  usually  had  to  deal  with, 
his  theoretical  thickness  ran  from  0.239 
to  0.451,  while  in  the  concluding  para- 
graphs of  the  paper  he  stated  that  the 
engineer  must  work  between  the  limits 
of  ^  the  thickness  and  •£,  which  seemed 
to  agree  with  the  theoretical  thickness. 
The  general  conclusion  that  engineers 
must  work  between  J  and  .£  was  differ- 
ent from  the  practice  in  which  Mr.  Lewis 
had  been  trained,  and  he  had  therefore 
brought  a  diagram  (Fig.  43)  of  a  retain- 
ing wall  constructed  according  to  Mr. 
Brunei's  rules.  Of  course  Mr.  Brunei, 
who  had  to  carry  out  very  great  works, 
modified  his  rules  to  suit  the  circum- 
stances ;  but  the  diagram  represented 
his  standard  section  of  wall  such  as  was 


137 

constructed  at  Lord  Hill's  land  in  the 
early  days  of  the  Great  Western  railway, 
and  at  the  Britain  Ferry  docks  two  years 
before  his  death.  It  would  be  seen  that 


Fig.43 


8cale,lC  feet  =  1  inch. 

the  dimensions  and  peculiarities  of  that 
wall  differed  very  much  from  those  given 
in  the  paper.  In  the  first  place  the  wall 
had  an  average  batter  of  1  in  5,  and  at 
the  top  a  batter  of  1  in  10.  Batter 


138 

was  a  point  on  which  Mr.  Brunei  al- 
ways insisted,  and  Mr.  Lewis  was  a  little 
surprised  that  the  author  seemed  to  treat 
it  with  so  much  indifference.  He  was 
evidently  aware  of  its  value,  because  in 
the  early  part  of  the  paper  he  mentioned 
a  wall  with  a  batter  of  1  in  5,  and  a 
thickness  of  1  foot,  which  he  said  was^ 
equivalent  to  a  vertical  wall  of  1  foot  9 
inches.  Now  anything  that  was  equiva- 
lent to  an  increase  of  the  original  value 
of  73  per  cent,  was  well  worthy  of  con- 
sideration. Mr.  Brunei's  custom  was  to 
curve  the  face  of  the  wall.  The  radius 
was  150  feet  in  the  case  of  a  30  feet  wall, 
or  five  times  the  height.  The  thickness 
was  -J-  to  J-  the  height.  The  counterforts 
were  2  feet  6  inches  thick,  and  placed  10 
feet  apart  from  center  to  center,  but 
were  omitted  in  good  clay  cuttings.  In 
the  case  of  docks  sometimes  there  was 
a  difficulty,  in  consequence  of  the  neces- 
sity of  having  the  top  more  upright, 
and  at  Britain  Ferry  docks  the  radius 
was  reduced  by  nearly  one-half.  Mr. 
Brunei,  too,  was  in  ihe  habit  of  building 


139 

behind  what  he  called  sailing  courses 
and  the  projections  in  Fig.  43  were  1 
foot  3  inches.  In  the  case  of  embank- 
ments the  wall  was  supported  by  earth 
carefully  punned  against  it  and  against 
the  sailing  courses,  thereby  adding  con- 
siderably to  the  weight  that  had  to  be 
overturned  when  pressure  came  from  be- 
hind. Then  his  rule  for  thickness  was 
i,  which  was  below  the  minimum  given 
by  the  author.  There  were  a  number  of 
such  walls  at  Paddington,  Bath,  Ply- 
mouth, Briton  Ferry,  30  feet  high  and  5 
feet  thick,  and  generally  of  nearly  the 
same  thickness  at  the  top  as  at  the  bot- 
tom. Another  point  Mr.  Brunei  was- 
particular  about  was  that  the  footings 
were  made  square  to  the  batter,  and 
when  the  ground  was  not  good  consider- 
ably larger  footings  were  introduced. 
At  Briton  Ferry  a  2- feet  lining  of  con- 
crete was  employed  at  some  places  for 
watertightness.  Concrete  was  not  then 
in  such  general  use  as  it  was  at  present, 
Of  course  when  exceptional  ground  was 
met  with  it  was  dealt  with  exceptionally. 


140 

At  a  tunnel  on  the  Wilts,  Somerset,  and 
Weymouth  railway,  some  heavy  ground 
had  been  found ;  the  tunnel  mouth  was 
in  a  60-feet  cutting,  a  retaining  wall  30 
feet  high  was  built,  and  the  top  was 
sloped  back  at  |  to  1,  with  a  2-feet  cov- 
ering of  masonry,  and  the  wall  was  built 
precisely  of  the  dimensions  represented  by 
Fig.  43 ;  but  as  the  ground  was  heavy,  the 
batter,  instead  of  being  1  in  5,  was  1  in 
4,  and  that  was  the  only  alteration.  That 
wall  was  builfc  in  1854,  had  never  given 
any  trouble,  and  was  standing  at  the  pres- 
ent moment.  It  seemed  to  him  that  Mr. 
Brunei,  forty  years  ago,  came  nearer  to 
the  teaching  of  the  experiments  and  of 
the  reasoning  in  the  paper,  than  the  au- 
thor had  ventured  to  do  in  his  own  prac- 
tice. 

Mr.  J.  B.  EEDMAN  observed  that  the 
author  had  undoubtedly  filled  a  void  in 
the  literature  of  engineering;  for,  not- 
withstanding the  great  experience  that 
most  of  the  members  of  the  Institution 
had  of  such  catastrophes  as  those  which 
had  been  referred  to,  it  was  only  human 


141 

like  that  they  had  not  been  often  record- 
ed by  the  designers  of  the  works.  Those 
who  constituted  what  was  now  a  select 
minority  of  the  Institution  would  re- 
member the  partial  failures  of  Mr.  Bob- 
ert  Stephenson's  retaining  walls  in  the 
Euston  cutting  of  what  was  then  the 
London  and  Birmingham,  and  now  the 
London  and  North-Western  railway. 
Those  partial  failures  were  met  by  over- 
head horizontal  girder  struts  supporting 
the  walls,  and  it  was  rather  curious  that, 
notwithstanding  all  the  experience  that 
had  been  since  gained,  in  a  large  number 
of  instances,  in  metropolitan  railways, 
the  overhead  girder  had  been,  as  it  were, 
the  natural  sequence  of  what  might  be 
termed  the  unretaining  wall.  There  was 
one  circumstance  which  very  much  com- 
plicated the  question  of  the  direct  lateral 
thrust  of  earthwork  upon  a  retaining 
wall,  and  which  rather  curiously  had  not 
been  mentioned  by  the  author.  It  was 
incidentally  referred  to  in  the  latter  part 
of  the  paper  where  the  author  said 
French  engineers,  in  designing  a  wall  at 


142 

Marseilles,  made  the  width  of  the  base 
58  per  cent,  of  the  vertical  height,  in 
consequence  of  the  dip  of  the  strata  be- 
ing towards  the  wall.  In  a  large  num- 
ber of  cases  of  the  failures  of  retaining 
walls  in  open  cuttings  near  London,  he 
thought  it  would  be  found  that  the  fail- 
ure was  entirely  on  one  side.  Where 
the  dip  of  the  strata  was  towards  the 
cutting,  and  more  especially  if  there 
were  lamince  of  clay,  the  superimposed 
strata  often  struck  near  the  base  of  the 
wall ;  and  a  retaining  wall  on  that  side 
not  only  had  to  support  the  normal  lat- 
eral thrust  of  the  mass  of  earthwork  im- 
mediately behind,  but  it  had  also  a  long 
wedge-like  piece  of  earth  impinging 
against  the  earth  at  the  back  of  the  wall, 
so  that  in  many  cases  the  thrust  on  the 
wall  at  the  one  side  must  be  something 
like  double  the  amount  that  it  was  on 
the  other ;  because  on  the  other  side,  the 
dip  being  away  from  the  wall,  the  wall 
was  subject  only  to  the  lateral  thrust  of 
the  earthwork  in  its  rear.  The  author 
had  stated  that  the  failures  of  many 


dock  walls  did  not  illustrate  entirely  the 
ordinary  lateral  thrust  of  earthwork ;  but 
Mr.  Redman  thought  thafc  such  cases  as 
the  failure  of  the  walls  constructed  by 
the  late  Mr.  G.  P.  Bidder,  Past-Presi- 
dent Inst.  C.E.,  at  the  Blackwall  entrance 
to  the  Victoria  docks,  the  partial  failure 
of  the  same  engineer's  walls  in  the  en- 
largement of  the  Surrey  Docks,  the  simi- 
lar catastrophe  at  the  Victoria  dock,  Hull, 
in  the  work  designed  by  the  late  Mr. 
John  Hartley,  and  possibly  also  a  similar 
movement  in  the  South  West  India  Dock 
wall,  were  all  clearly  attributable  to  lat- 
eral thrust.  It  might  be  said  that  the 
foundation  was  not  taken  down  deep 
enough,  and  consequently  the  wall  did 
not  resist  that  thrust,;  but  having  had  a 
somewhat  extended  and  varied  experi- 
ence for  a  great  number  of  years,  he  * 
certainly  was  not  prepared  to  indorse  the 
dogma  that  a  dock  wall  or  a  river  wall 
must  necessarily  be  so  strong  as  to  resist 
a  head  of  water,  or  in  width  at  the  base 
equal  to  one-half  the  height.  In  the 
first  place,  the  water  ought  not  to  be  al- 


144 

lowed  to  come  behind  the  wall.  There 
were  exceptional  cases,  perhaps,  where 
that  could  hardly  be  avoided;  but  it 
seemed  to  him  that  laying  down  such  a 
tenet  was  a  premium  for  loose  engineer- 
ing, imperfect  supervision,  and  lavish 
expenditure.  He  had  himself,  in  the 
lower  reaches  of  the  Thames,  erected 
some  of  the  heaviest  embankment  walls 
on  the  river,  where  the  thickness  was 
only  J  of  the  vertical  height.  It  was  true 
that  the  walls  were  founded  on  the  best 
possible  foundation — Thames  ballast — 
and  it  was  done  as  tide  work ;  and  the 
greatest  possible  care  was  also  taken  to 
keep  the  backing  up  to  the  same  level  as 
the  wall,  and  indeed  rather  above  the 
wall.  In  fact,  the  great  mistake  in  re- 
taining walls  was  the  imperfect  supervi- 
sion exercised  over  the  backing.  If  the 
backing  were  put  in  with  tolerably  fair 
material  in  thin  horizontal  layers  and 
brought  up  in  that  way,  the  lateral 
thrust  was  reduced  to  a  very  small  mat- 
ter. The  author  had  stated  that  the  de- 
cayed timber  wharves  on  the  Thames  and 


145 

in  other  neighborhoods  showed  that  the 
lateral  thrust  must  be  over-estimated ; 
but  it  should  be  remarked  that  the  skin 
might  be  stripped  off  the  face  of  the 
earthwork,  assuming  that  no  water  was 
coming  against  it,  and  it  would  stand, 
because  from  the  length  of  time  and 
consolidation  of  material,  there  was  no 
lateral  thrust.  The  example  quoted  of 
the  breakwater  at  St.  Katherine's,  Jer- 
sey, appeared  to  be  a  case  in  point.  He 
had  nothing  to  do  with  the  inception  or 
execution  of  that  work;  but  he  thought 
the  wall  might  be  taken  down  and  the 
heart  of  the  pier  would  still  stand.  He 
would  refer  to  two  great  Metropolitan 
failures  which  were  well  known,  and 
which  might  be  interesting  in  illustra- 
tion of  this  subject.  One  was  that  of 
Greenwich  Pier  and  the  other  of  the. 
Island  Lead  Works.  The  Greenwich 
Pier  was  constructed  nearly  half  a  cen- 
tury ago  from  the  design  of  a  local  archi- 
tect, Mr.  Martyr.  It  was  one  of  the 
heaviest  embankments  on  the  Thames ; 
it  had  the  greatest  depth  of  water  up  to 


146 

it,  and  it  was,  being  in  the  hollow  of  the 
reach,  subject  to  every  condition  of 
weather.  The  base  was  formed  by  cast- 
iron  piling  and  cast-iron  sheeting  be- 
tween, constituting  a  half-tide  dam,  and 
concrete  was  got  in  behind.  Upon  the 
top  of  the  concrete  there  were  large  6-inch 
York  landings  and  a  very  solid,  heavy 
brick  wall.  There  were  also  outer  piles, 
and  the  work  was  constructed  in  the  best 
possible  way.  The  case  was  somewhat 
complicated  by  the  fact  that  a  large 
amount  of  land-water  came  down  and  a 
large  amount  of  spring  water.  There 
was  a  common  sewer  running  through 
the  heart  of  the  work,  and  a  large  tidal 
reservoir  for  the  Ship  Hotel.  The  whole 
of  that  work,  with  the  exception  of  the 
two  returns  and  quoins  and  a  small  por- 
tion in  front  of  the  Ship  Hotel,  slipped 
into  the  river  during  the  night  some 
forty  years  back.  The  late  Mr.  Chad- 
wick,  who  built  the  Hungerford  suspen- 
sion bridge,  entered  into  a  contract  to 
restore  the  work  on  his  own  plan,  acting 
as  engineer  and  contractor,  and  he  re- 


147 

stored  the  portion  that  had  failed  with 
timber-bearing  piles  and  a  solidly-con- 
structed brick  wall.  Shortly  after  the 
demise  of  Mr.  Chadwick,  the  restored 
portion  showed  signs  of  failure,  and  Mr. 
Redman  was  called  in  by  the  Pier  Direct- 
orate, and  the  matter  resulted  in  a  law- 
suit, and  a  large  sum  of  money  was  ob- 
tained in  compensation.  All  he  did  was 
to  bleed  the  pier  by  inserting  a  cast- 
iron  pipe  with  a  self-acting  flap  at  the 
eastern  end,  and  to  remove  and  sub- 
stitute with  better  material  some  part  of 
the  backing.  He  proposed  driving  land- 
tie  piles  at  the  back  and  some  in  front ; 
but  on  consideration  with  the  Director- 
ate, it  was  thought  that  driving  piles 
might  be  a  ticklish  operation.  That  was 
twenty  years  ago,  and  up  to  the  present 
time  the  work  had  remained  in  the  same 
state.  It  had  settled  somewhat  at  the 
eastern  end,  and  there  were  reopened 
fissures  in  front,  so  that  the  movement 
had  not  altogether  ceased.  The  wall  of 
the  Island  Lead  Works  designed  by  the 
late  Mr.  K.  Sibley,  M.  Inst.  C.E.,  was  the 


148 

pioneer  of  cast-iron  wharfing ;  and  from 
the  fact  of  the  Limehouse  cut  having 
been  deepened  too  close  up  to  it,  the 
wall  failed.  As  the  author  had  said,  that 
case  did  not  illustrate  the  absolute  lat- 
eral pressure  of  earthwork,  because  this 
work,  as  long  as  it  was  not  meddled  with, 
stood  satisfactorily.  The  leaseholders 
called  in  Mr.  Redman  on  that  occasion, 
and  the  freeholder  consulted  Mr.  Bate- 
man,  Past-President  Inst.  C.E.,  and  the 
late  Mr.  N.  Beardmore,  M.  Inst.  C.E., 
Tery  wisely — to  avoid  a  lawsuit — con- 
structed a  wall  deeper  down,  to  their 
satisfaction. 

Mr.  W.  ATKINSON  agreed  with  Mr. 
Lewis's  remarks  with  respect  to  the 
large  amount  of  masonry  or  brickwork 
that  the  author  had  introduced  in  the 
cases  of  the  metropolitan  railways.  He 
had  been  much  struck  with  the  propor- 
tion of  J  of  the  height  for  the  mean 
thickness  of  a  wall ;  but  looking  at  the 
diagrams,  and  taking  into  consideration 
what  he  had  seen  of  the  work,  there  was 
a  very  good  explanation.  It  struck  him 


149 

that  on  the  Metropolitan  railway,  where 
property  was  so  valuable,  the  batter 
which  the  late  Mr.  Brunei  introduced  of 
1  in  5  would  be  extremely  inconvenient ; 
either  the  roadway  would  have  to  be 
narrowed,  or  a  great  deal  more  property 
would  have  to  be  taken,  than  would  be 
otherwise  necessary.  No  doubt  the 
author  would  be  able  to  say  whether  that 
had  any  influence  in  the  carrying  out  of 
the  work.  Then  with  regard  to  the  gen- 
eral question  of  the  walls  and  their  fail- 
ure due  to  bad  foundations,  it  struck 
him  that  the  two  things  should  be  en- 
tirely separate ;  that  the  foundation 
should  be  treated  as  a  foundation,  and 
that  having  been  made  sufficiently  strong, 
a  properly  proportioned  wall  should  be 
placed  upon  it.  He  rather  gathered^ 
from  the  paper  that  the  two  points  had 
been  taken  as  a  whole,  and  that  the  au- 
thor meant,  "I  have  a  bad  foundation, 
and  I  will  make  the  whole  to  stand."  If 
that  were  so,  it  would  have  been  better 
policy  to  have  made  a  foundation  of  con- 
crete, and  then  put  a  wall  sufficiently 


150 

strong.  With  regard  to  the  question  of 
theoretical  calculation,  there  was  a 
French  formula  which  agreed  remark- 
ably with  what  might  be  considered  the 
ordinary  practice.  He  himself  had  put 
up  a  good  many  walls,  not  perhaps  as 
distinct  retaining  walls,  but  in  connec- 
tion with  bridges  on  48  miles  of  the  Mid 
Wales  railway,  and  he  had  found  practi- 
cally that  the  ^  of  the  height  for  the 
mean  thickness  stood  perfectly  well.  In 
that  case,  it  was  to  be  borne  in  mind  that 
there  were  two  elements  in  addition  to 
the  theoretical  calculation,  namely,  the 
projection  of  the  footings  where  there 
was  so  much  leverage,  which  was  not 
taken  into  account  in  the  calculation, 
and  the  weight  of  the  earth  resting  on 
the  projections  or  steppings  at  the  back 
of  the  wall,  Fig.  44.  That,  of  course, 
aided  the  wall  very  materially  ;*  in  fact,  it 
might  be  called  so  much  masonry  saved. 
At  all  events,  if  merely  the  theoretical 
thickness  of  the  wall  was  given,  then, 
with  the  projections  of  the  footings,  and 
the  weight  of  earth  on  the  steppings, 


151 

there  was  a  very  good  margin  of  safety; 
and  in  that  way  the  wall  was  erected 
with  J-^,  or  33  per  cent,  less  than  the 
dimension  advocated  by  the  author,  and 


Fig.44 


Scale  JG  feet  =  1  inch. 


was  a  good  and  sufficient  wall.  One 
point  with  regard  to  walls  was  brought 
to  his  notice  when  in  Canada,  namely, 
the  thickening  of  the  top  to  resist  frost. 
In  ordinary  circumstances  the  practice 


152 

would  be  to  put  about  2  feet  at  the  top, 
and  then  about  9  feet  down  a  projection 
of  9  inches,  and  so  on ;  but  in  Canada, 
on  account  of  the  penetration  of  the 
frost,  it  had  been  found  necessary  to 
make  the  top  of  the  wall  much  thicker 
than  was  the  practice  in  England. 

Mr.  H.  LAW  desired  to  add  his  testi- 
mony to  the  great  value  of  the  facts  laid 
before  the  Institution.  It  was  upon  such 
facts,  the  result  of  actual  experience,  that 
the  most  valuable  data  were  formed.  In 
the  early  part  of  the  paper  the  author 
had  pointed  out  that  the  formula  usually 
adopted — Coulomb's — did  not  give  the 
results  which  were  obtained  when  loosely 
heaped  materials  were  placed  at  the  back 
of  the  wall;  but  a  little  consideration 
would  show  that  that  formula  never  was 
intended  to  apply  to  such  cases.  Cou- 
lomb's theorem  distinctly  took  into  ac- 
count the  adhesiveness  or  coherence  of 
the  ground,  and  then  determined,  de- 
pending upon  the  line  on  which  the 
ground  separated,  what  the  amount  of 
pressure  would  be;  and  the  value  to  the 


153 

engineer  was,  that  it  determined  what 
was  the  maximum  which  that  pressure 
could  be.  Putting  w=ihe  weight  of  a 
cubic  foot  of  the  soil  in  Ibs.,  h  =  the 
height  of  the  wall  in  feet,  r  =  the  limit- 
ing angle  of  resistance  of  the  soil,  s  = 
the  angle  between  the  line  at  which  the 
soil  separated  and  the  horizontal,  and  P 
=  the  horizontal  pressure  in  Ibs.  of  the 
soil  against  the  wall,  then  Coulomb's 
theorem  might  be  thus  expressed: 

P=:-^—  .  cot  s.  tan(s— r). 
2 

Now   in   the   case  of   a  fluid,  r,  or   the 
limiting   angle   of   resistance,   vanished, 
and  consequently  the  result  was  that  the 
co-tangent   of   s  into   the   tangent  of   s 
became  equal  to  unity,  and 
_rf 
"  2  ' 

When  the  ground  was  sufficiently  co- 
herent to  stand  vertically,  then  the  angle 
of  separation  being  90°  the  co-tangent 
of  s  became  nothing,  and  the  pressure 
became  nothing.  When  the  line  of 


154 

separation  coincided  with  the  limiting 
angle  of  resistance  or  r,  that  was  to  say, 
when  there  was  a  mass  of  earth  suffi- 
ciently coherent  not  to  break  of  itself, 
and  lying  upon  a  bed  which  happened  to 
be  at  the  limiting  angle  of  resistance,  the 
tendency  of  the  earth  to  slide  was  exact- 
ly overcome  by  its  friction,  and  r  being 
equal  to  s,  the  tangent  vanished,  and  P 
again  became  nothing.  Now,  between 
tho^e  two  values  there  was  a  certain 
angle  at  which,  if  the  ground  separated, 
it  would  produce  the  maximum  pressure, 
and  that  was  given  by  Coulomb's  theorem, 
which  proved  that  when  the  line  of  sep- 
aration bisected  the  angle  made  by  the 
limiting  angle  of  resistance  with  the 
vertical,  then  cots=tan(s—  r),  and 


and  the  maximum  pressure  was  obtained. 
The  great  value  of  the  formula  was  to 
show,  with  a  given  weight  of  earth  and  a 
given  limiting  angle  of  resistance,  what 
the  maximum  pressure  was.  It  could 


155 

not  exceed  the  value  expressed  by  making 
s  half  the  angle  between  the  limiting 
angle  of  resistance  and  the  verticaJ.  This 
formula  could  not  be  applied  in  the  case 
of  loose  materials,  as  sand  and  gravel, 
because  it  was  impossible  for  such  ma- 
terials to  stand  at  any  other  than  than 
their  limiting  angle  of  resistance ;  and 
under  such  circumstances  there  would  be 
upon  the  wall  only  a  comparatively  small 
pressure,  due  to  the  unbalanced  weight 
which  remained  from  the  efforts  of  the 
sand  and  the  gravel  to  roll  down  upon 
itself.  He  wished  to  direct  attention  to 
one  or  two  interesting  exemplifications  of 
excessive  pressure  which  were  met  with 
in  the  works  for  the  Thames  tunnel.  The 
Rotherhithe  shaft,  50  feet  in  diameter, 
was  built  upon  the  surface  and  sunk  by 
excavating  beneath.  That  operation  was* 
successful  until  a  depth  of  40  feet  was 
reached,  and  then,  although  the  exterior 
surface  had  been  made  perfectly  smooth 
by  being  rendered,  it  became  earth- 
bound,  and  notwithstanding  the  earth 
w  as  excavated  to  a  depth  of  2  feet  round 


156 

the  whole  margin,  and  50,000  bricks  were 
placed  upon  the  top  as  a  load,  making  the 
total  weight  1,100  tons,  and  water  was 
allowed  to  rise  inside,  the  shaft  refused 
to  sink  any  farther.  Now,  taking  the 
weight  of  the  ground  at  120  Ibs.  per 
cubic  foot,  which  was  about  what  it  was 
on  the  average,  and  taking  the  coefficient 
of  friction  at  0  67,  it  would  be  found  that 
a  limiting  angle  of  resistance  of  about 
31°  15',  and  a  line  of  fracture  of  about 
27°  30',  would  show,  by  Coulomb's 
theorem,  that  the  shaft  would  be  bound, 
and  therefore  the  practical  result  was 
quite  in  accordance  with  the  pressure 
given  by  the  formula.  The  author  had 
mentioned  a  case  of  some  heavy  clay 
which  had  a  pressure  equivalent  to  a 
fluid  pressure  of  107  Ibs.,  and  if  that  clay 
was  taken  as  having  a  limiting  angle  of 
resistance  of  about  5°  or  1  in  10,  and  the 
weight  was  assumed  to  be  130  Ibs.  per 
cubic  foot — which  clay  of  that  descrip- 
tion might  very  well  have — the  formula 
would  give  107  Ibs.  for  the  fluid  pressure. 
He  therefore  thought  these  circumstances 


157 

fully  showed  that  where  ground  was  co- 
herent and  adhesive,  Coulomb's  theorem 
applied.  In  the  progress  of  the  Thames 
tunnel  there  had  been  some  remarkable 
cases  of  excessive  pressure,  where  of 
course  the  weight  of  the  water  was  super- 
added  to  that  of  the  ground.  He  knew 
many  instances  of  poling  boards,  3  feet 
in  length,  6  inches  wide,  and  3  inches 
thick,  supported  by  two  poling  screws 
bearing  against  cast  iron  plates,  being 
split  lengthwise  by  the  pressure  of  the 
earth  against  the  outer  surface. 

Mr.  E.  A.  BEENAYS  said  the  inconsis- 
tencies alluded  to  in  the  paper  tended  to 
make  it  still  more  interesting  than  it 
otherwise  would  have  been.  There  were 
few  engineers  who  had  carried  out 
works,  but  were  conscious  of  inconsist- 
encies in  their  own  practice  and  theories* 
The  author  had  quoted  M.  Voisin  Bey, 
the  distinguished  French  engineer,  as 
saying  that  he  had  rarely  seen  a  long 
wall  straight,  and  Mr.  Bernays'  expe- 
rience fully  confirmed  •  that  view.  When 
it  was  straight  the  chances  were  there 


158 

was  a  superabundance  of  material  to  keep 
it  so.  If  it  was  run  fine,  as  the  calcula- 
tions advised,  the  chances  were  50  to  1 
against  having  a  straight  wall.  With  re- 
gard to  Mr.  Brunei's  section  of  wall,  no 
doubt  if  it  had  a  good  foundation  it  was 
very  strong  for  the  material  in  it.  It 
not  only  had  a  rising  abutment  to  bring 
the  pressure  down  upon  the  foundation, 
but  it  had  counterforts,  which  added 
greatly  to  the  strength  .of  the  wallr 
although  of  late  they  had  gone  out  of 
fashion.  He  considered  it  was  nearer  10 
feet  at  the  base  than  5  feet,  as,  if  the 
counterforts  were  10  feet  apart,  the  wall 
was,  practically,  a  solid  wall.  If  made  of 
concrete  instead  of  brickwork,  it  would 
probably  be  found  better  to  make  it  solid 
at  once.  The  batter  added  consider- 
ably to  the  strength,  but  it  was  not 
without  practical  disadvantages.  The 
greater  the  batter  the  greater  the  disad- 
vantage. The  tendency  of  the  batter  was 
to  throw  the  side  of  a  vessel  farther  away 
from  the  wall  than  need  be,  and  to  entail 
cranes  with  longer  jibs,  as  well  as  the  use 


159 

of  much  larger  fenders.  Iron  ships  were 
now  all  covered  with  anti-fouling  com- 
position, which  might  easily  be  scraped 
off.  With  all  its  disadvantages  he  would 
rather  have  a  smaller  batter  for  practical 
purposes  when  ships  were  to  lie  along- 
side the  wall.  He  had  seen  a  wall  of 
this  section  in  Woolwich  Dockyard  (built, 
he  believed,  by  Sir  John  Bennie,  Past- 
President  Inst.  C.E.),  partially  pulled 
down  and  refaced  by  the  late  Mr.  James 
Walker,  Past-President  Inst.  C.E.,  for 
the  purpose  of  deepening  the  dock.  It  was 
about  30, feet  deep,  and  was  increased  to 
about  38  feet  by  putting  a  thin  wall  in  front 
of  it.  In  pulling  clown  such  walls  he  had 
always  found  that  the  backing  in  settling 
hung  upon  the  set  off,  and  he  had  seen 
holes  under  the  backing  large  enough  for 
a  man  to  creep  in.  He  would  not  say 
that  they  were  objectionable  in  other 
respects,  but  he  preferred  a  battered 
back  to  a  retaining  wall  to  square  sets 
off.  The  author  had  alluded  to  a  wall 
that  he  was  building,  and  had  character- 
ized it  as  <c  exceptionably  heavy."  But 


160 

for  that  expression  he  would  have  been 
quite  content  to  sit  still :  he  hoped  to  be 
able  to  show  that  the  exceptionable 
heaviness  was  justified  by  the  exceptional 
circumstances  under  which  it  was  being 
built.  He  did  not  think  much  of  experi- 
ments with  peas  and  pea-gravel,  and  bits 
of  board  a  foot  square  when  he  had  to 
deal  with  big  walls.  The  author  stated 
(p.  47)  that  "  experience  has  shown  that 
a  wall  %J  of  the  height  in  thickness,  and 
flattering  1  inch  or  2  inches  per  foot  on 
the  face,  possesses  sufficient  stability 
when  the  backing  and  foundation  are  both 
favorable."  Unfortunately  for  dock  en- 
gineers it  rarely  happened  that  either 
the  foundation  or  the  backing  was 
favorable,  and  it  was  still  rarer  to  find 
both  favorable.  This  fact  made  the  in- 
consistencies that  really  showed  the 
thoughtful  way  in  which  the  paper  had 
been  written.  There  was  no  attempt  to 
square  theory  with  practice ;  but  the 
author  had  candidly  pointed  out  where 
theory  broke  down  in  referring  to  the 
retaining  walls  of  the  Metropolitan  Rail 


161 

way,  and  at  the  approach  to  the  Euston 
Station,  and  in  other  instances.  He  agreed 
with  the  author  that  the  Sheerness  river 
wall  had  perhaps  a  greater  moment  of  sta- 
bility than  any  other  wall  in  the  world. 
The  section  assumed  that  the  pile  foun- 
dation would  stand,  though  he  doubted 
its  stability;  but  if  it  would  stand,  half 
the  thickness  of  the  wall  would  have  been 
ample.  He  did  not  know  sufficient  of 
the  nature  of  the  subsoil  at  Sheerness  to 
be  able  to  decide  the  point.  He  had  been 
told  that  in  many  cases  the  piles  were 
40  feet  long ;  and  a  few  years  ago,  when 
a  new  caisson  was  put  in  the  basin  at  the 
yard,  there  was  great  fear  lest  it  would 
come  forward  when  the  water  was  let 
out.  That  was  merely  an  instance  of  the 
cases  where  provision  must  be  made  for 
very  different  calculations  from  those 
which  were  set  out  in  any  table.  He 
quite  agreed  with  the  author  that  no 
calculations  would  meet  cases  where  the 
work  was  exceptionally  difficult.  In  most 
instances,  engineers  were  called  upon  to 
make  docks  and  other  greafc  works  in  the 


162 

worst  kinds  of  soils,  such  as  estuaries, 
beds  of  rivers,  or  in  deep  alluvial  de- 
posits. The  reason  that  he  strengthened 
the  wall  at  Chatham  was  because  the 
original  design  showed  symptoms  of 
weakness,  and  several  of  the  walls 
yielded  about  10  or  12  inches.  He  did 
not  say  that  that  was  entirely  the  fault 
of  the  walls,  because  the  foundation  was 
far  from  satisfactory,  and  there  was  a 
decided  forward  movement  of  the  piles  ; 
but  it  was  evident  that  the  wall,  for  the 
greater  part  of  the  area,  was  not  at  any 
rate  too  otrong  for  the  work.  When, 
however,  he  came  to  the  east  end  of  the 
works,  where  he  had  to  build  a  wall 
1,050  feet  long  without  a  single  break, 
and  with  35  feet  depth  of  soft  mud  to  ex- 
cavate through,  it  was  absolutely  neces- 
sary to  strengthen  the  wall,  and  it  was 
decided  to  build  it  entirely  of  concrete, 
in  order  to  be  able  to  give  the  additional 
strength  without  additional  cost.  He 
was  asked  some  years  ago  what  angle 
this  mud  would  assume  at  rest,  and  the 
answer  he  gave  was  that  it  would  not  lie 


163 

flat.  The  basin  at  Chatham  was  being 
built  in  an  old  arm  of  the  river  Medway, 
and  the  basin  generally  stood  in  the 
middle  of  the  river  bed.  On  each  side  of 
it  the  mud  was  35  feet  deep  on  the  aver- 
age, and  in  some  cases  the  distance  to  be 
filled  in  with  backing  was  500  feet.  The 
whole  of  that  backing  had  to  be  laid  on 
this  sliding  mud,  which  brought  pressure 
on  the  wall  in  a  way  far  beyond  anything 
he  had  ever  seen  allowed  for  in  any  cal- 
culation. If  he  understood  correctly, 
Mr  Giles  had  thought  it  necessary  to 
provide,  not  only  for  the  backing,  but 
for  a  pressure  of  water  nearly  as  high  as 
the  water  in  the  dock.  He  did  not  agree 
with  Mr.  Redman  that  it  was  possible  to 
get  the  walls  built  up  so  as  to  prevent 
water  percolating.  At  Chatham  there 
was  a  starring  level  of  the  water  in  tile 
district,  and  wherever  excavations  were 
made  to  that  depth  water  was  found. 
The  bottom  of  the  basin  was  20  or  25  feet 
below  that  level,  and  the  water  exerted  a 
pressure  just  as  if  there  was  an  ocean  of 
that  depth  behind  it.  Then  it  was  some- 


164 

times  necessary  to  put  heavy  buildings, 
as  at  the  Victoria  Dock  Extension,  where 
large  sheds  loaded  with  heavy  goods 
were  placed  from  100  to  150  feet  from  the 
wall.  No  one  would  say  that  such  sheds 
would  not  exercise  a  great  pressure  on  the 
adjacent  wall.  He  would  be  happy  to 
show  the  author  the  wall  at  the  Chatham 
Dockyard  Extension,  and  abide  by  that 
gentleman's  judgment,  whether  "  the  ex- 
ceptionally heavy  wall "  was  not  neces 
sary  to  meet  the  peculiar  conditions  of 
the  case. 

Mr.  A.  GILES,  after  what  Mr.  Bernays 
had  said  about  the  pressure  of  mud  be- 
hind dock  walls,  thought  he  was  quite 
justified  in  adhering  to  the  assertion  he 
made  many  years  ago,  that  a  dock  wall 
ought  to  be  strong  enough  to  carry  a 
head  of  water  behind  it  equal  to  its 
height.  He  cordially  joined  in  thanking 
the  author  for. the  paper,  but  he  consid- 
ered it  would  have  been  better  described 
as  "  On  the  Stability  of  Retaining  Walla." 
The  author  had  given  many  examples  of 
dock  and  retaining  walls,  but  after 


165 

throwing  over  the  theoretical  calculation 
as  to 'the  pressure  of  earth  against  a  wall, 
he  said  that  in  ninety- nine  cases  out  of  a 
hundred  walls  failed  from  faulty  founda- 
tions, and  not  from  want  of  strength  in 
themselves.  The  various  diagrams  af- 
forded rather  congratulatory  evidence  of 
his  own  theory,  that  practically  all  the 
thick  walls  had  stood,  and  most  of  the 
thin  walls  had  given  way.  Referring  to 
the  old  Southampton  dock  wall,  mention- 
ed as  having  been  built  40  years  ago,  that 
had  only  a  thickness  of  32  per  cent,  at 
the  base,  but  with  the  counterforts  it  was 
35  per  cent.  That  wall  had  been  pushed 
forward,  but  it  never  came  down ;  but  it 
was  saved  by  taking  out  the  wet  soil  at 
the  top  and  covering  the  top  by  a  timber 
platform.  Another  wall  which  he  had 
built  had  been  referred  to.  That  had  a 
thickness  of  45  per  cent,  at  the  base,  and 
an  average  thickness  of  41  per  cent. 
Surely  that  wall  ought  to  be  strong 
enough  to  resist  not  only  the  pressure 
of  water  behind  it,  but  even  the  pressure 
of  mud  that  would  not 

OF  THE 


166 

It  had  not  stood  without  moving — not 
from  any  want  of  strength  in  the  wall, 
but  simply  from  the  want  of  adhesion  in 
the  foundation.  At  Whitehaven  the 
thickness  of  the  wall  was  37  per  cent,  at 
the  base,  and  the  mean  thickness  was  31 
per  cent.,  and  it  had  stood.  At  Avon- 
mouth  these  values  were  respectively  59 
per  cent,  and  42  per  cent.  There  was  a 
very  fine  example  at  Carlingf ord  of  a  wall 
with  the  base  only  32  per  cent,  thick,  and 
a  mean  thickness  of  24  per  cent.;  but 
what  could  be  said  about  a  wall  at  Sheer- 
ness  with  a  height  of  40  feet  and  a  base 
of  43  feet?  It  was  stated  that  that  wall 
had  not  moved,  and  Mr.  Bernays  had  con- 
tended that  it  ought  not  to  move ;  but 
he  did  not  think  any  engineer  of  the  pres- 
ent day  would  dare  to  design,  or  con- 
template building,  a  wall  of  that  char- 
acter, because  a  wall  of  similar  height  in 
ordinary  ground  could  be  built  for  £60  a 
yard,  while  that  wall  would  cost  £300  a 
yard.  In  many  instances  it  was  not  the 
inherent  weakness  of  the  walls  that 
caused  them  to  fall,  but  the  slip  at  the 


167 

bottom,  and  that  was  shown  in  Fig.  17  by 
the  necessity  which  arose  for  thickening 
the  wall  so  as  to  make  the  strength  as  62 
to  24.  After  all,  the  wall  required  still 
further  strengthening  by  putting  but- 
tresses in  front  of  it.  The  conviction 
he  had  arrived  at  was,  that  it  was  not 
generally  the  fault  of  the  wall  that  caused 
the  failure  ;  but  the  fault  of  the  founda- 
tion—not only  that  the  foundation  was 
not  wide  enough  to  give  sufficient  hold 
on  the  ground,  but  that  there  was  not 
sufficient  footing  in  front  of  the  wall  to 
enable  the  soil  upon  which  the  wall  rested 
to  sustain  the  weight.  It  was  the  same 
as  if  a  cliff,  30  or  40  feet  high,  were  put 
on  tender  soil.  The  soil  would  not  be 
strong  enough  to  bear  it,  and  conse- 
quently the  edge  of  the  cliff  would  settle 
into  the  soil,  the  soil  would  burst  up  in ' 
front,  and  the  pressure  from  behind  would 
then  make  itself  felt.  He  had  seen  that 
process  take  place  in  a  wall  which  he  had 
constructed,  and  it  was  only  saved  by 
putting  buttresses  in  front  of  the  foot- 
ings. Something  had  been  said  in  the 


168 

paper  about  allowing  a  margin  for  con- 
tingencies. In  that  matter  every  engi- 
neer must  decide  for  himself ;  but  he 
thought  that  from  ^  to  ^  was  rather  a 
large  margin,  and  he  would  suggest  that 
the  thickness  of  a  retaining  wall  J  of  the 
height  would  be,  in  nine  cases  out  of  ten, 
ample  to  resist  the  backward  pressure ; 
but  he  would  insist  upon  having  a  large 
buttress  in  front  of  the  foundation,  car- 
ried down  as  deep  as  the  lowest  founda- 
tions of  the  wall.  He  was  at  a  loss  to 
imagine  what  the  extraordinary  projec- 
tion in  front  of  the  wall  at  Marseilles 
(Fig.  25)  meant.  It  might  be  that  it  was 
intended  to  hold  the  bottom  down ;  and 
it  was  in  that  direction  he  would  recom- 
mend retaining  walls  should  be  strength- 
ened. A  remark  had  been  made  about 
the  necessity  of  having  the  upper  part  of 
dock  walls  nearly  perpendicular,  because 
of  the  friction  of  ships  rubbing  against 
them,  and  the  inconvenience  of  ships 
lying  at  some  distance  from  the  quay  at 
the  coping  level.  That  was  perfectly 
true,  and  he  believed  it  had  been  a  com- 


169 

rnon  practice,  in  designing  walls  where 
there  was  a  curved  batter,  to  make  the 
center  of  the  curve  level  with  the  coping, 
by  which  a  certain  depth  of  almost  per- 
pendicular work  was  obtained  from  the 
coping  level.  He  believed  that  was  the 
correct  principle  ;  but  he  would  urge  par- 
ticularly that,  in  making  dock  walls,  the 
foundation  should  be  much  wider  than 
they  were  in  general,  and  that  the  bulk 
of  the  buttress  should  be  in  front  of  the 
face  of  the  wall,  and  not  behind.  In  all 
walls  the  excavation  at  the  bottom  should 
be  carried  down  perpendicularly,  with  as 
little  disturbance  of  the  soil  as  possible ; 
because  in  excavating  the  work,  it  was 
better  to  fill  up  the  void  so  made,  that  there 
should  be  no  tendency  to  slip  after  the 
wall  was  put  in.  There  was  another 
point  which  he  thought  was  not  suffi- 
ciently considered  by  engineers  in  de- 
signing dock  walls.  They  were  apt,  when 
the  excavation  had  been  carried  out,  to 
think  that  they  had  got  a  good  founda- 
tion ;  but  he  cordially  agreed  with  the 
author  when  he  used  the  word  "lubricat- 


170 

ing."  Notwithstanding  what  Mr.  Red- 
man had  said,  he  did  not  think  it  was 
possible  to  keep  water  from  getting  be- 
hind a  dock  wall :  he  believed  there  was 
a  point  at  which  water  would  always  be 
found  :  it  would  get  up  from  the  bottom 
or  through  the  wall  somewhere  ;  and  that 
being  so,  he  thought  that  all  the  soil 
upon  which  the  wall  stood  must  be  sod- 
dened  and  lubricated  to  a  certain  extent. 
He  knew  of  instances  where  walls  had 
stood  for  many  years ;  but  all  at  once  the 
moment  of  lubrication  had  arrived,  and 
they  slipped  in.  He  could  only  account  for 
it  by  supposing  that  there  was  a  tendency 
on  the  part  of  walls  to  get  surrounded 
with  water,  by  which  they  became  of  less 
specific  gravity,  or  that  the  soil  got  satu- 
rated, and  therefore  less  able  to  bear  the 
load  put  upon  it.  He  would  therefore 
urge  upon  all  his  professional  brethren 
who  had  the  conduct  of  dock  works  to 
look  particularly  to  the  front  of  the  walls 
to  ensure  their  stability. 

Mr.  W.  E.  BOUSFIELD  desired  to  make 
one  or  two  remarks,  from  the  theoretical 


171 

standpoint  which  the  author  had  depre- 
cated. He  referred  to  a  point  in  which 
theory  and  practice  would  agree,  viz.,  as 
to  the  effect  of  water  behind  a  retaining 
wall.  If  there  was  an  interstice  of  even 
an  inch  the  effect  on  the  wall  would  be 
exactly  the  same  as  if  the  whole  ocean 
were  behind  it;  therefore,  a  dock  wall 
should  be  made  to  withstand  a  pressure 
equal  to  the  hydrostatic  pressure  due  to 
a  head  of  water  of  the  height  of  the  wall. 
He  wished  to  ask  if  the  author  could  ex- 
plain, somewhat  more  at  length,  the 
effect  of  lateral  pressure  in  General  Bur- 
goyne's  experiments,  for  he  did  not  think 
the  remarks  were  quite  sound.  If  the 
lateral  pressure  of  the  ground,  consist- 
ing, say,  of  loose  rubble,  was  greater 
than  the  hydrostatic  pressure,  the  fact  of 
water  being  admitted  would  not  make 
the  slightest  difference,  because  the 
water  pressed  equally  on  the  earth  and 
on  the  wall  in  opposite  directions,  so 
that  the  earth  would  be  kept  back  by  the 
pressure,  and  the  difference  between  the 
lateral  pressure  and  the  hydrostatic 


172 

pressure  would  be  exerted  by  the  earth 
on  the  wall.  The  only  effect  would  be 
in  the  distribution  of  the  pressure,  which 
instead  of  being  taken  by  the  points  of 
stone  alone,  would  be  distributed  by  the 
water  over  the  whole  wall.  If  the  lateral 
pressure  of  the  soil  was  less  than  the 
hydrostatic  pressure,  then  of  course,  if 
water  was  admitted  behind,  it  would  ex- 
ert upon  the  soil  a  force  greater  than  the 
pressure  of  the  soil  on  the  wall ;  there- 
fore, supposing  the  soil  were  rigid,  the 
lateral  pressure  of  the  soil  would  be 
kept  entirely  off  the  wall.  Of  course,  in 
practice,  there  were  many  points  at  which 
the  pressure  was  excessive,  so  that,  on 
the  whole,  the  maximum  pressure  to  be 
provided  for  would  generally  be  rather 
more  than  the  hydrostatic  pressure. 

Mr.  E.  BENEDICT  described  a  retaining 
wall  (Fig.  45)  lately  put  up  at  Kyde.  The 
ground  was  sidelong  and  at  the  foot  of  a 
clay  hill,  the  strata  dipping  towards  the 
work,  and  with  a  heavy  building  close  to 
it.  By  cutting  a  trench  and  filling  it  as 
soon  as  possible  with  solid  concrete  in 


173 

Portland  cement  carried  up  to  the  sur- 
face, the  clay,  which  weathered  rapidly 
when  expose^  to  the  air,  was  covered 


without  delay,  the  concrete  became  ag- 
glomerated with  the  clay  at  the  back, 
and  did  not  allow  any  percolation  of 
water.  The  excavation  in  front  of  the 


174 

wall  then  proceeded  without  any  move- 
ment of  the  ground  occurring,  and  he 
thought  that  none  would  take  place. 
Eventually  a  covered  way  was  formed  on 
the  lower  side  of  the  wall,  the  arch  of 
which  was  designed  so  as  to  form  a  con- 
tinuous lying  buttress. 

Mr.  B.  BAKER,  in  reply,  observed  that 
he  agreed,  to  some  extent,  with  almost 
everything  that  had  been  said  in  the  dis- 
cussion, and  he  considered  that  the  criti- 
cism had  been  very  fair.  He  was  glad 
indeed  that  he  had  elicited  so  many  valu- 
able opinions  on  the  subject.  Mr.  Airy 
spoke  about  the  difference  in  the  cohesion 
of  different  clays.  He  had  noticed  the 
same  thing  himself,  not  merely  in  differ- 
ent clays  but  in  the  same  clay.  A  rail- 
way cutting  often  refused  to  stand  at  a 
less  slope  than  4  to  1,  and  yet  the  same 
clay,  after  being  tempered  a  little,  might 
be  found  in  an  adjoining  brick  kiln 
standing  with  a  vertical  face.  He  had 
nothing  to  say  with  regard  to  Mr.  Ver- 
non  Harcourt's  comments,  except  that  he 
agreed  with  almost  everything  that  gentle- 


175 

man  had  said.  Mr.  Barry  had  made  some 
sensible  remarks  about  the  advantage  of 
using  struts  to  retaining  walls,  and  he 
thought  it  would  be  a  good  thing,  in 
many  cases,  to  imitate  the  old  architects 
of  cathedrals,  and  substitute  flying 
buttresses  for  a  heavy  mass  of  materials 
Some  time  ago  he  designed  some  very 
cheap  sheds  upon  that  principle,  in 
which  the  roofs  were  light  concrete 
arches  supported  by  flying  buttresses. 
Mr.  Barry  had  referred  to  the  cracks 
at  the  angles  of  some  of  the  piers 
of  the  Metropolitan  District  railway  re- 
taining walls.  He  was  satisfied  that 
these  did  not  arise  from  pressure,  but 
from  chemical  action,  because  they  oc- 
curred only  in  the  case  of  certain  bricks, 
and  he  knew  where  the  bricks  came  from, , 
and  had  every  reason  to  mistrust  them. 
He  had  sometimes  found  scaling  occur  all 
over  the  face  of  a  wall,  though  of  course 
the  angle  was  always  the  weakest  point, 
and  nature  always  tried  to  round  off  an 
angle,  as  might  be  seen  in  Cleopatra's 
Needle,  where  there  was  no  square  angle. 


176 

Mr.  Lewis  had  described  a  wall  designed 
by  the  late  Mr.  Brunei,  but  he  did  not 
approve  of  it,  for  reasons  that  had  been 
set  forth  by  Mr.  Bernays.  He  himself 
had  found  exactly  the  same  thing,  name- 
ly, that  in  pulling  down  work  where 
there  had  been  the  slightest  settlement, 
the  earth  at  the  back  did  not  rest  on  the 
offset?,  indeed,  not  infrequently,  a  man 
could  push  in  his  arm  between  the  offset 
and  the  filling.  It  was  therefore  idle  to 
maintain,  as  Professor  Bankine  and  oth- 
ers did,  that  the  earthwork  resting  on 
the  offset  was  as  good  as  so  much  mason- 
ry. There  was  no  economy  in  putting 
in  the  offsets,  and  he  attributed  the  sta- 
bility of  apparently  light  walls  so  con- 
structed to  the  pressure  of  the  counter- 
forts and  the  good  quality  of  the  back- 
ng.  Mr.  Eedmanhad  directed  attention 
to  the  fact  that  there  was  an  increased 
thrust  when  the  ground  at  the  back  was 
sloping.  No  doubt  that  was  so ;  and  a 
case  of  that  sort  was  referred  to  in  the 
discussion  on  Mr.  Constable's  paper  at 
the  American  Society  of  Engineers, 


177 

where  the  ground  at  the  back  was  slop- 
ing rock.  When  the  wall  was  first  put 
up  and  the  backing  was  filled  in,  the 
whole  mass  came  forward  in  consequence 
of  the  wedge  of  earth  sliding  down  the 
surface  of  the  rock.  The  masonry  was 
pulled  down,  the  rock  cut  in  steps,  and 
the  wall  rebuilt  of  the  same  thickness. 
Pig  iron  to  the  extent  of  55  tons  to  the 
lineal  foot  was  then  placed  behind  the 
wall,  and  it  stood  perfectly  well,  though 
the  thickness  was  less  than  30  per  cent, 
of  the  height.  That  was  sufficient  evi- 
dence of  the  importance  of  stepping  the 
ground  at  the  back.  Mr.  Atkinson  said 
he  thought  it  would  be  better  to  make 
the  foundation  satifactory  first  and  then 
to  build  a  thin  wall  on  the  top  of  it.  At 
page  43  of  the  paper  that  point  was  al- 
luded to  and  the  answer  given.  Mr 
Law  had  submitted  a  formula,  and  drawn 
deductions  from  it,  which  he  could  not 
follow ;  but  it  seemed  to  him  that  the 
contention  was  that  a  loose  material  ex- 
erted less  thrust  on  theVall  than  a  more 
compact  material,  and  that  Coulomb's 


178 

theory  was  not  applicable  to  loose  soil. 
He  did  not  agree  with  that  view  in  theory, 
and  Lieutenant  Hope's  experiments 
showed  that  that  was  aiot  so  in  practice, 
at. least  on  a  small  scale.  Lieutenant 
Hope  placed  a  board  behind  the  pressure 
board  at  such  an  angle  as  to  include 
Coulomb's  wedge  of  maximum  thrust 
between  the  two  boards,  and  found  that 
the  lateral  pressure  was  quite  as  much 
when  the  board  was  at  the  slope  of  re- 
pose, 1£  to  1,  as  when  it  was  at  ha]f  the 
angle.  There  was  hardly  any  difference 
whether  the  board  was  horizontal  or  at  a 
slope  of  ^  to  1,  or  at  any  intermediate 
slope.  Then  it  had  been  remarked  with 
regard  to  one  of  his  examples,  in  which 
the  stability  of  the  wall  was  equal  to  the 
fluid  pressure  of  107  Ibs.  per  cubic  foot, 
that  theory  would  indicate  the  pressure 
to  be  about  that  amount ;  but  a  state- 
ment in  the  paper  did  not  seem  to  have 
been  noticed,  that  the  wall  never  had 
that  pressure  on  it,  but  failed  by  sliding 
forward.  Of  course  it  might  have  slid 
forward  with  a  pressure  of  40  Ibs^  per 


179 

cubic  foot,  but  since  the  struts  had 
been  put  in,  there  was  not  the  slightest 
indication  of  movement,  and  therefore 
the  moment  of  stability  could  not  have 
been  deficient.  Mr.  Bernays  seemed  to 
imagine  that  the  expression  "  excessively 
heavy"  reflected  on  the  design  of  the 
Chatham  dock  wall,  but  the  intention 
was  the  reverse.  He  entirely  approved 
of  it,  and  considered  that  it  was  a  well 
designed  and  creditable  engineering 
work  in  every  respect.  It  was  one  that 
he  should  imitate.  His  contention 
throughout  the  paper  was  that  formulae 
did  not  apply  to  such  works,  and  al- 
though he  began  the  paper  with  a  dia- 
gram he  set  it  up  merely  in  order  to 
knock  it  over.  Mr.  Giles  considered 
that  instead  of  the  limits  of  J  to  ^  of 
the  height  for  the  thickness  of  a  retain- 
ing wall,  J  should  be  the  limit  with  a 
buttress  in  front  of  the  toe  ;  but  he  did 
not  think  that  that  was  the  practice 
which  had  been  followed  in  the  South- 
ampton Dock  Extension,  where  the  limit, 
he  believed,  was  nearer  %~  than  J — 45  per 


180 

cent.  The  curious  slope  projection  in 
Fig.  25  was  really  an  apron  to  protect 
the  foundation,  which  was  of  clay.  The 
clay  was  very  hard  when  laid  bare,  and  a 
sort  of  shield  was  put  there  to  prevent 
its  softening.  He  believed  the  same 
thing  had  been  recommended  by  a  com- 
mittee of  engineers  in  the  case  of  the 
Belfast  dock,  where  the  wall  failed  ;  but 
it  was  applied  too  late,  or  the  conditions 
were  different,  because  the  wall  came  for- 
ward notwithstanding. 


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